What is the speed of light?

The speed of light is the speed limit of the universe. Or is it?

graphic representing the speed of light showing lines of light of different colors; blue, green, yellow and white.

What is a light-year?

  • Speed of light FAQs
  • Special relativity
  • Faster than light
  • Slowing down light
  • Faster-than-light travel

Bibliography

The speed of light traveling through a vacuum is exactly 299,792,458 meters (983,571,056 feet) per second. That's about 186,282 miles per second — a universal constant known in equations as "c," or light speed. 

According to physicist Albert Einstein 's theory of special relativity , on which much of modern physics is based, nothing in the universe can travel faster than light. The theory states that as matter approaches the speed of light, the matter's mass becomes infinite. That means the speed of light functions as a speed limit on the whole universe . The speed of light is so immutable that, according to the U.S. National Institute of Standards and Technology , it is used to define international standard measurements like the meter (and by extension, the mile, the foot and the inch). Through some crafty equations, it also helps define the kilogram and the temperature unit Kelvin .

But despite the speed of light's reputation as a universal constant, scientists and science fiction writers alike spend time contemplating faster-than-light travel. So far no one's been able to demonstrate a real warp drive, but that hasn't slowed our collective hurtle toward new stories, new inventions and new realms of physics.

Related: Special relativity holds up to a high-energy test

A l ight-year is the distance that light can travel in one year — about 6 trillion miles (10 trillion kilometers). It's one way that astronomers and physicists measure immense distances across our universe.

Light travels from the moon to our eyes in about 1 second, which means the moon is about 1 light-second away. Sunlight takes about 8 minutes to reach our eyes, so the sun is about 8 light minutes away. Light from Alpha Centauri , which is the nearest star system to our own, requires roughly 4.3 years to get here, so Alpha Centauri is 4.3 light-years away.

"To obtain an idea of the size of a light-year, take the circumference of the Earth (24,900 miles), lay it out in a straight line, multiply the length of the line by 7.5 (the corresponding distance is one light-second), then place 31.6 million similar lines end to end," NASA's Glenn Research Center says on its website . "The resulting distance is almost 6 trillion (6,000,000,000,000) miles!"

Stars and other objects beyond our solar system lie anywhere from a few light-years to a few billion light-years away. And everything astronomers "see" in the distant universe is literally history. When astronomers study objects that are far away, they are seeing light that shows the objects as they existed at the time that light left them. 

This principle allows astronomers to see the universe as it looked after the Big Bang , which took place about 13.8 billion years ago. Objects that are 10 billion light-years away from us appear to astronomers as they looked 10 billion years ago — relatively soon after the beginning of the universe — rather than how they appear today.

Related: Why the universe is all history

Speed of light FAQs answered by an expert

We asked Rob Zellem, exoplanet-hunter and staff scientist at NASA's Jet Propulsion Lab, a few frequently asked questions about the speed of light. 

Rob Zellem

Dr. Rob Zellem is a staff scientist at NASA's Jet Propulsion Laboratory, a federally funded research and development center operated by the California Institute of Technology. Rob is the project lead for Exoplanet Watch, a citizen science project to observe exoplanets, planets outside of our own solar system, with small telescopes. He is also the Science Calibration lead for the Nancy Grace Roman Space Telescope's Coronagraph Instrument, which will directly image exoplanets. 

What is faster than the speed of light?

Nothing! Light is a "universal speed limit" and, according to Einstein's theory of relativity, is the fastest speed in the universe: 300,000 kilometers per second (186,000 miles per second). 

Is the speed of light constant?

The speed of light is a universal constant in a vacuum, like the vacuum of space. However, light *can* slow down slightly when it passes through an absorbing medium, like water (225,000 kilometers per second = 140,000 miles per second) or glass (200,000 kilometers per second = 124,000 miles per second). 

Who discovered the speed of light?

One of the first measurements of the speed of light was by Rømer in 1676 by observing the moons of Jupiter . The speed of light was first measured to high precision in 1879 by the Michelson-Morley Experiment. 

How do we know the speed of light?

Rømer was able to measure the speed of light by observing eclipses of Jupiter's moon Io. When Jupiter was closer to Earth, Rømer noted that eclipses of Io occurred slightly earlier than when Jupiter was farther away. Rømer attributed this effect due the time it takes for light to travel over the longer distance when Jupiter was farther from the Earth. 

How did we learn the speed of light?

As early as the 5th century, Greek philosophers like Empedocles and Aristotle disagreed on the nature of light speed. Empedocles proposed that light, whatever it was made of, must travel and therefore, must have a rate of travel. Aristotle wrote a rebuttal of Empedocles' view in his own treatise, On Sense and the Sensible , arguing that light, unlike sound and smell, must be instantaneous. Aristotle was wrong, of course, but it would take hundreds of years for anyone to prove it. 

In the mid 1600s, the Italian astronomer Galileo Galilei stood two people on hills less than a mile apart. Each person held a shielded lantern. One uncovered his lantern; when the other person saw the flash, he uncovered his too. But Galileo's experimental distance wasn't far enough for his participants to record the speed of light. He could only conclude that light traveled at least 10 times faster than sound.

In the 1670s, Danish astronomer Ole Rømer tried to create a reliable timetable for sailors at sea, and according to NASA , accidentally came up with a new best estimate for the speed of light. To create an astronomical clock, he recorded the precise timing of the eclipses of Jupiter's moon , Io, from Earth . Over time, Rømer observed that Io's eclipses often differed from his calculations. He noticed that the eclipses appeared to lag the most when Jupiter and Earth were moving away from one another, showed up ahead of time when the planets were approaching and occurred on schedule when the planets were at their closest or farthest points. This observation demonstrated what we today know as the Doppler effect, the change in frequency of light or sound emitted by a moving object that in the astronomical world manifests as the so-called redshift , the shift towards "redder", longer wavelengths in objects speeding away from us. In a leap of intuition, Rømer determined that light was taking measurable time to travel from Io to Earth. 

Rømer used his observations to estimate the speed of light. Since the size of the solar system and Earth's orbit wasn't yet accurately known, argued a 1998 paper in the American Journal of Physics , he was a bit off. But at last, scientists had a number to work with. Rømer's calculation put the speed of light at about 124,000 miles per second (200,000 km/s).

In 1728, English physicist James Bradley based a new set of calculations on the change in the apparent position of stars caused by Earth's travels around the sun. He estimated the speed of light at 185,000 miles per second (301,000 km/s) — accurate to within about 1% of the real value, according to the American Physical Society .

Two new attempts in the mid-1800s brought the problem back to Earth. French physicist Hippolyte Fizeau set a beam of light on a rapidly rotating toothed wheel, with a mirror set up 5 miles (8 km) away to reflect it back to its source. Varying the speed of the wheel allowed Fizeau to calculate how long it took for the light to travel out of the hole, to the adjacent mirror, and back through the gap. Another French physicist, Leon Foucault, used a rotating mirror rather than a wheel to perform essentially the same experiment. The two independent methods each came within about 1,000 miles per second (1,609 km/s) of the speed of light.

Another scientist who tackled the speed of light mystery was Poland-born Albert A. Michelson, who grew up in California during the state's gold rush period, and honed his interest in physics while attending the U.S. Naval Academy, according to the University of Virginia . In 1879, he attempted to replicate Foucault's method of determining the speed of light, but Michelson increased the distance between mirrors and used extremely high-quality mirrors and lenses. Michelson's result of 186,355 miles per second (299,910 km/s) was accepted as the most accurate measurement of the speed of light for 40 years, until Michelson re-measured it himself. In his second round of experiments, Michelson flashed lights between two mountain tops with carefully measured distances to get a more precise estimate. And in his third attempt just before his death in 1931, according to the Smithsonian's Air and Space magazine, he built a mile-long depressurized tube of corrugated steel pipe. The pipe simulated a near-vacuum that would remove any effect of air on light speed for an even finer measurement, which in the end was just slightly lower than the accepted value of the speed of light today. 

Michelson also studied the nature of light itself, wrote astrophysicist Ethan Siegal in the Forbes science blog, Starts With a Bang . The best minds in physics at the time of Michelson's experiments were divided: Was light a wave or a particle? 

Michelson, along with his colleague Edward Morley, worked under the assumption that light moved as a wave, just like sound. And just as sound needs particles to move, Michelson and Morley and other physicists of the time reasoned, light must have some kind of medium to move through. This invisible, undetectable stuff was called the "luminiferous aether" (also known as "ether"). 

Though Michelson and Morley built a sophisticated interferometer (a very basic version of the instrument used today in LIGO facilities), Michelson could not find evidence of any kind of luminiferous aether whatsoever. Light, he determined, can and does travel through a vacuum.

"The experiment — and Michelson's body of work — was so revolutionary that he became the only person in history to have won a Nobel Prize for a very precise non-discovery of anything," Siegal wrote. "The experiment itself may have been a complete failure, but what we learned from it was a greater boon to humanity and our understanding of the universe than any success would have been!"

Special relativity and the speed of light

Einstein's theory of special relativity unified energy, matter and the speed of light in a famous equation: E = mc^2. The equation describes the relationship between mass and energy — small amounts of mass (m) contain, or are made up of, an inherently enormous amount of energy (E). (That's what makes nuclear bombs so powerful: They're converting mass into blasts of energy.) Because energy is equal to mass times the speed of light squared, the speed of light serves as a conversion factor, explaining exactly how much energy must be within matter. And because the speed of light is such a huge number, even small amounts of mass must equate to vast quantities of energy.

In order to accurately describe the universe, Einstein's elegant equation requires the speed of light to be an immutable constant. Einstein asserted that light moved through a vacuum, not any kind of luminiferous aether, and in such a way that it moved at the same speed no matter the speed of the observer. 

Think of it like this: Observers sitting on a train could look at a train moving along a parallel track and think of its relative movement to themselves as zero. But observers moving nearly the speed of light would still perceive light as moving away from them at more than 670 million mph. (That's because moving really, really fast is one of the only confirmed methods of time travel — time actually slows down for those observers, who will age slower and perceive fewer moments than an observer moving slowly.)

In other words, Einstein proposed that the speed of light doesn't vary with the time or place that you measure it, or how fast you yourself are moving. 

Therefore, objects with mass cannot ever reach the speed of light. If an object ever did reach the speed of light, its mass would become infinite. And as a result, the energy required to move the object would also become infinite: an impossibility.

That means if we base our understanding of physics on special relativity (which most modern physicists do), the speed of light is the immutable speed limit of our universe — the fastest that anything can travel. 

What goes faster than the speed of light?

Although the speed of light is often referred to as the universe's speed limit, the universe actually expands even faster. The universe expands at a little more than 42 miles (68 kilometers) per second for each megaparsec of distance from the observer, wrote astrophysicist Paul Sutter in a previous article for Space.com . (A megaparsec is 3.26 million light-years — a really long way.) 

In other words, a galaxy 1 megaparsec away appears to be traveling away from the Milky Way at a speed of 42 miles per second (68 km/s), while a galaxy two megaparsecs away recedes at nearly 86 miles per second (136 km/s), and so on. 

"At some point, at some obscene distance, the speed tips over the scales and exceeds the speed of light, all from the natural, regular expansion of space," Sutter explained. "It seems like it should be illegal, doesn't it?"

Special relativity provides an absolute speed limit within the universe, according to Sutter, but Einstein's 1915 theory regarding general relativity allows different behavior when the physics you're examining are no longer "local."

"A galaxy on the far side of the universe? That's the domain of general relativity, and general relativity says: Who cares! That galaxy can have any speed it wants, as long as it stays way far away, and not up next to your face," Sutter wrote. "Special relativity doesn't care about the speed — superluminal or otherwise — of a distant galaxy. And neither should you."

Does light ever slow down?

Light in a vacuum is generally held to travel at an absolute speed, but light traveling through any material can be slowed down. The amount that a material slows down light is called its refractive index. Light bends when coming into contact with particles, which results in a decrease in speed.

For example, light traveling through Earth's atmosphere moves almost as fast as light in a vacuum, slowing down by just three ten-thousandths of the speed of light. But light passing through a diamond slows to less than half its typical speed, PBS NOVA reported. Even so, it travels through the gem at over 277 million mph (almost 124,000 km/s) — enough to make a difference, but still incredibly fast.

Light can be trapped — and even stopped — inside ultra-cold clouds of atoms, according to a 2001 study published in the journal Nature . More recently, a 2018 study published in the journal Physical Review Letters proposed a new way to stop light in its tracks at "exceptional points," or places where two separate light emissions intersect and merge into one.

Researchers have also tried to slow down light even when it's traveling through a vacuum. A team of Scottish scientists successfully slowed down a single photon, or particle of light, even as it moved through a vacuum, as described in their 2015 study published in the journal Science . In their measurements, the difference between the slowed photon and a "regular" photon was just a few millionths of a meter, but it demonstrated that light in a vacuum can be slower than the official speed of light. 

Can we travel faster than light?

— Spaceship could fly faster than light

— Here's what the speed of light looks like in slow motion

— Why is the speed of light the way it is?

Science fiction loves the idea of "warp speed." Faster-than-light travel makes countless sci-fi franchises possible, condensing the vast expanses of space and letting characters pop back and forth between star systems with ease. 

But while faster-than-light travel isn't guaranteed impossible, we'd need to harness some pretty exotic physics to make it work. Luckily for sci-fi enthusiasts and theoretical physicists alike, there are lots of avenues to explore.

All we have to do is figure out how to not move ourselves — since special relativity would ensure we'd be long destroyed before we reached high enough speed — but instead, move the space around us. Easy, right? 

One proposed idea involves a spaceship that could fold a space-time bubble around itself. Sounds great, both in theory and in fiction.

"If Captain Kirk were constrained to move at the speed of our fastest rockets, it would take him a hundred thousand years just to get to the next star system," said Seth Shostak, an astronomer at the Search for Extraterrestrial Intelligence (SETI) Institute in Mountain View, California, in a 2010 interview with Space.com's sister site LiveScience . "So science fiction has long postulated a way to beat the speed of light barrier so the story can move a little more quickly."

Without faster-than-light travel, any "Star Trek" (or "Star War," for that matter) would be impossible. If humanity is ever to reach the farthest — and constantly expanding — corners of our universe, it will be up to future physicists to boldly go where no one has gone before.

Additional resources

For more on the speed of light, check out this fun tool from Academo that lets you visualize how fast light can travel from any place on Earth to any other. If you’re more interested in other important numbers, get familiar with the universal constants that define standard systems of measurement around the world with the National Institute of Standards and Technology . And if you’d like more on the history of the speed of light, check out the book " Lightspeed: The Ghostly Aether and the Race to Measure the Speed of Light " (Oxford, 2019) by John C. H. Spence.

Aristotle. “On Sense and the Sensible.” The Internet Classics Archive, 350AD. http://classics.mit.edu/Aristotle/sense.2.2.html .

D’Alto, Nick. “The Pipeline That Measured the Speed of Light.” Smithsonian Magazine, January 2017. https://www.smithsonianmag.com/air-space-magazine/18_fm2017-oo-180961669/ .

Fowler, Michael. “Speed of Light.” Modern Physics. University of Virginia. Accessed January 13, 2022. https://galileo.phys.virginia.edu/classes/252/spedlite.html#Albert%20Abraham%20Michelson .

Giovannini, Daniel, Jacquiline Romero, Václav Potoček, Gergely Ferenczi, Fiona Speirits, Stephen M. Barnett, Daniele Faccio, and Miles J. Padgett. “Spatially Structured Photons That Travel in Free Space Slower than the Speed of Light.” Science, February 20, 2015. https://www.science.org/doi/abs/10.1126/science.aaa3035 .

Goldzak, Tamar, Alexei A. Mailybaev, and Nimrod Moiseyev. “Light Stops at Exceptional Points.” Physical Review Letters 120, no. 1 (January 3, 2018): 013901. https://doi.org/10.1103/PhysRevLett.120.013901 . 

Hazen, Robert. “What Makes Diamond Sparkle?” PBS NOVA, January 31, 2000. https://www.pbs.org/wgbh/nova/article/diamond-science/ . 

“How Long Is a Light-Year?” Glenn Learning Technologies Project, May 13, 2021. https://www.grc.nasa.gov/www/k-12/Numbers/Math/Mathematical_Thinking/how_long_is_a_light_year.htm . 

American Physical Society News. “July 1849: Fizeau Publishes Results of Speed of Light Experiment,” July 2010. http://www.aps.org/publications/apsnews/201007/physicshistory.cfm . 

Liu, Chien, Zachary Dutton, Cyrus H. Behroozi, and Lene Vestergaard Hau. “Observation of Coherent Optical Information Storage in an Atomic Medium Using Halted Light Pulses.” Nature 409, no. 6819 (January 2001): 490–93. https://doi.org/10.1038/35054017 . 

NIST. “Meet the Constants.” October 12, 2018. https://www.nist.gov/si-redefinition/meet-constants . 

Ouellette, Jennifer. “A Brief History of the Speed of Light.” PBS NOVA, February 27, 2015. https://www.pbs.org/wgbh/nova/article/brief-history-speed-light/ . 

Shea, James H. “Ole Ro/Mer, the Speed of Light, the Apparent Period of Io, the Doppler Effect, and the Dynamics of Earth and Jupiter.” American Journal of Physics 66, no. 7 (July 1, 1998): 561–69. https://doi.org/10.1119/1.19020 . 

Siegel, Ethan. “The Failed Experiment That Changed The World.” Forbes, April 21, 2017. https://www.forbes.com/sites/startswithabang/2017/04/21/the-failed-experiment-that-changed-the-world/ . 

Stern, David. “Rømer and the Speed of Light,” October 17, 2016. https://pwg.gsfc.nasa.gov/stargaze/Sun4Adop1.htm . 

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Vicky Stein

Vicky Stein is a science writer based in California. She has a bachelor's degree in ecology and evolutionary biology from Dartmouth College and a graduate certificate in science writing from the University of California, Santa Cruz (2018). Afterwards, she worked as a news assistant for PBS NewsHour, and now works as a freelancer covering anything from asteroids to zebras. Follow her most recent work (and most recent pictures of nudibranchs) on Twitter. 

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Life's Little Mysteries

Can anything travel faster than the speed of light?

Does it matter if it's in a vacuum?

Artist's impression of beams of light

In 1676, by studying the motion of Jupiter's moon Io, Danish astronomer Ole Rømer calculated that light travels at a finite speed. Two years later, building on data gathered by Rømer, Dutch mathematician and scientist Christiaan Huygens became the first person to attempt to determine the actual speed of light, according to the American Museum of Natural History in New York City. Huygens came up with a figure of 131,000 miles per second (211,000 kilometers per second), a number that isn't accurate by today's standards — we now know that the speed of light in the "vacuum" of empty space is about 186,282 miles per second (299,792 km per second) — but his assessment showcased that light travels at an incredible speed.

According to Albert Einstein 's theory of special relativity , light travels so fast that, in a vacuum, nothing in the universe is capable of moving faster. 

"We cannot move through the vacuum of space faster than the speed of light," confirmed Jason Cassibry, an associate professor of aerospace engineering at the Propulsion Research Center, University of Alabama in Huntsville.

Question answered, right? Maybe not. When light is not in a vacuum, does the rule still apply?

Related: How many atoms are in the observable universe?

"Technically, the statement 'nothing can travel faster than the speed of light' isn't quite correct by itself," at least in a non-vacuum setting, Claudia de Rham, a theoretical physicist at Imperial College London, told Live Science in an email. But there are certain caveats to consider, she said. Light exhibits both particle-like and wave-like characteristics, and can therefore be regarded as both a particle (a photon ) and a wave. This is known as wave-particle duality.

If we look at light as a wave, then there are "multiple reasons" why certain waves can travel faster than white (or colorless) light in a medium, de Rham said. One such reason, she said, is that "as light travels through a medium — for instance, glass or water droplets — the different frequencies or colors of light travel at different speeds." The most obvious visual example of this occurs in rainbows, which typically have the long, faster red wavelengths at the top and the short, slower violet wavelengths at the bottom, according to a post by the University of Wisconsin-Madison . 

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When light travels through a vacuum, however, the same is not true. "All light is a type of electromagnetic wave, and they all have the same speed in a vacuum (3 x 10^8 meters per second). This means both radio waves and gamma rays have the same speed," Rhett Allain, a physics professor at Southeastern Louisiana University, told Live Science in an email.

So, according to de Rham, the only thing capable of traveling faster than the speed of light is, somewhat paradoxically, light itself, though only when not in the vacuum of space. Of note, regardless of the medium, light will never exceed its maximum speed of 186,282 miles per second.

Universal look

According to Cassibry, however, there is something else to consider when discussing things moving faster than the speed of light.

"There are parts of the universe that are expanding away from us faster than the speed of light, because space-time is expanding," he said. For example, the Hubble Space Telescope recently spotted 12.9 billion year-old light from a distant star known as Earendel. But, because the universe is expanding at every point, Earendel is moving away from Earth and has been since its formation, so the galaxy is now 28 billion light years away from Earth.

In this case, space-time is expanding, but the material in space-time is still traveling within the bounds of light speed.

Related: Why is space a vacuum?

Diagram of the visible color spectrum

So, it's clear that nothing travels faster than light that we know of, but is there any situation where it might be possible? Einstein's theory of special relativity, and his subsequent theory of general relativity, is "built under the principle that the notions of space and time are relative," de Rham said. But what does this mean? "If someone [were] able to travel faster than light and carry information with them, their notion of time would be twisted as compared to ours," de Rham said. "There could be situations where the future could affect our past, and then the whole structure of reality would stop making sense."

This would indicate that it would probably not be desirable to make a human travel faster than the speed of light. But could it ever be possible? Will there ever be a time when we are capable of creating craft that could propel materials — and ultimately humans — through space at a pace that outstrips light speed? "Theorists have proposed various types of warp bubbles that could enable faster-than-light travel," Cassibry said.

But is de Rham convinced?

"We can imagine being able to communicate at the speed of light with systems outside our solar system ," de Rham said. "But sending actual physical humans at the speed of light is simply impossible, because we cannot accelerate ourselves to such speed.

"Even in a very idealistic situation where we imagine we could keep accelerating ourselves at a constant rate — ignoring how we could even reach a technology that could keep accelerating us continuously — we would never actually reach the speed of light," she added. "We could get close, but never quite reach it."

Related: How long is a galactic year?

This is a point confirmed by Cassibry. "Neglecting relativity, if you were to accelerate with a rate of 1G [Earth gravity], it would take you a year to reach the speed of light. However, you would never really reach that velocity because as you start to approach lightspeed, your mass energy increases, approaching infinite. "One of the few known possible 'cheat codes' for this limitation is to expand and contract spacetime, thereby pulling your destination closer to you. There seems to be no fundamental limit on the rate at which spacetime can expand or contract, meaning we might be able to get around this velocity limit someday."

— What would happen if the speed of light were much lower?

— What if the speed of sound were as fast as the speed of light?

— How does the rubber pencil illusion work?

Allain is similarly confident that going faster than light is far from likely, but, like Cassibry, noted that if humans want to explore distant planets, it may not actually be necessary to reach such speeds. "The only way we could understand going faster than light would be to use some type of wormhole in space," Allain said. "This wouldn't actually make us go faster than light, but instead give us a shortcut to some other location in space."

Cassibry, however, is unsure if wormholes will ever be a realistic option.

"Wormholes are theorized to be possible based on a special solution to Einstein's field equations," he said. "Basically, wormholes, if possible, would give you a shortcut from one destination to another. I have no idea if it's possible to construct one, or how we would even go about doing it." Originally published on Live Science.

Joe Phelan

Joe Phelan is a journalist based in London. His work has appeared in VICE, National Geographic, World Soccer and The Blizzard, and has been a guest on Times Radio. He is drawn to the weird, wonderful and under examined, as well as anything related to life in the Arctic Circle. He holds a bachelor's degree in journalism from the University of Chester. 

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light travel fastest in vacuum

expanding universe light waves

How Light Travels: The Reason Why Telescopes Can See the Invisible Parts of Our Universe

Due to how light travels, we can only see the most eye-popping details of space—like nebulas, supernovas, and black holes—with specialized telescopes.

  • Our eyes can see only a tiny fraction of these wavelengths , but our instruments enable us to learn far more.
  • Here, we outline how various telescopes detect different wavelengths of light from space.

Light travels only one way: in a straight line. But the path it takes from Point A to Point B is always a waveform, with higher-energy light traveling in shorter wavelengths. Photons , which are tiny parcels of energy, have been traveling across the universe since they first exploded from the Big Bang . They always travel through the vacuum of space at 186,400 miles per second—the speed of light—which is faster than anything else.

Too bad we can glimpse only about 0.0035 percent of the light in the universe with our naked eyes. Humans can perceive just a tiny sliver of the electromagnetic spectrum: wavelengths from about 380–750 nanometers. This is what we call the visible part of the electromagnetic spectrum. The universe may be lovely to look at in this band, but our vision skips right over vast ranges of wavelengths that are either shorter or longer than this limited range. On either side of the visible band lies evidence of interstellar gas clouds, the hottest stars in the universe, gas clouds between galaxies , the gas that rushes into black holes, and much more.

electromagnetic spectrum the visible range shaded portion is shown enlarged on the right

Fortunately, telescopes allow us to see what would otherwise remain hidden. To perceive gas clouds between stars and galaxies, we use detectors that can capture infrared wavelengths. Super-hot stars require instruments that see short, ultraviolet wavelengths. To see the gas clouds between galaxies, we need X-ray detectors.

We’ve been using telescopes designed to reveal the invisible parts of the cosmos for more than 60 years. Because Earth’s atmosphere absorbs most wavelengths of light, many of our telescopes must observe the cosmos from orbit or outer space.

Here’s a snapshot of how we use specialized detectors to explore how light travels across the universe.

Infrared Waves

galaxy glass z13 through webb

We can’t see infrared waves, but we can feel them as heat . A sensitive detector like the James Webb Space Telescope can discern this thermal energy from far across the universe. But we use infrared in more down-to-Earth ways as well. For example, remote-control devices work by sending infrared signals at about 940 nanometers to your television or stereo. These heat waves also emanate from incubators to help hatch a chick or keep a pet reptile warm. As a warm being, you radiate infrared waves too; a person using night vision goggles can see you, because the goggles turn infrared energy into false-color optical energy that your eyes can perceive. Infrared telescopes let us see outer space in a similar way.

Astronomers began the first sky surveys with infrared telescopes in the 1960s and 1970s. Webb , launched in 2021, takes advantage of the infrared spectrum to probe the deepest regions of the universe. Orbiting the sun at a truly cold expanse—about one million miles from Earth—Webb has three infrared detectors with the ability to peer farther back in time than any other telescope has so far.

Its primary imaging device, the Near Infrared Camera (NIRCam), observes the universe through detectors tuned to incoming wavelengths ranging from 0.6 to 5 microns, ideal for seeing light from the universe’s earliest stars and galaxies. Webb’s Mid-Infrared Instrument (MIRI) covers the wavelength range from 5 to 28 microns, its sensitive detectors collecting the redshifted light of distant galaxies. Conveniently for us, infrared passes more cleanly through deep space gas and dust clouds, revealing the objects behind them; for this and many other reasons, the infrared spectrum has gained a crucial foothold in our cosmic investigations. Earth-orbiting satellites like NASA’s Wide Field Infrared Survey Telescope ( WFIRST ) observe deep space via longer infrared wavelengths, too.

Yet, when stars first form, they mostly issue ultraviolet light . So why don’t we use ultraviolet detectors to find distant galaxies? It’s because the universe has been stretching since its beginning, and the light that travels through it has been stretching, too; every planet, star, and galaxy continually moves away from everything else. By the time light from GLASS-z13—formed 300 million years after the Big Bang—reaches our telescopes, it has been traveling for more than 13 billion years , a vast distance all the way from a younger universe. The light may have started as ultraviolet waves, but over vast scales of time and space, it ended up as infrared. So, this fledgling galaxy appears as a red dot to NIRCam. We are gazing back in time at a galaxy that is rushing away from us.

Radio Waves

m87 supermassive black hole in polarised light

If we could see the night sky only through radio waves, we would notice swaths of supernovae , pulsars, quasars, and gassy star-forming regions instead of the usual pinprick fairy lights of stars and planets.

Tools like the Arecibo Observatory in Puerto Rico can do the job our eyes can’t: detect some of the longest electromagnetic waves in the universe. Radio waves are typically the length of a football field, but they can be even longer than our planet’s diameter. Though the 1,000-foot-wide dish at Arecibo collapsed in 2020 due to structural problems, other large telescopes carry on the work of looking at radio waves from space. Large radio telescopes are special because they actually employ many smaller dishes, integrating their data to produce a really sharp image.

Unlike optical astronomy, ground-based radio telescopes don’t need to contend with clouds and rain. They can make out the composition, structure, and motion of planets and stars no matter the weather. However, the dishes of radio telescopes need to be much larger than optical ones to generate a comparable image, since radio waves are so long. The Parkes Observatory’s dish is 64 meters wide, but its imaging is comparable to a small backyard optical telescope, according to NASA .

Eight different radio telescopes all over the world coordinated their observations for the Event Horizon Telescope in 2019 to put together the eye-opening image of a black hole in the heart of the M87 galaxy (above).

Ultraviolet Waves

sun in ultraviolet nasa image

You may be most familiar with ultraviolet, or UV rays, in warnings to use sunscreen . The sun is our greatest local emitter of these higher-frequency, shorter wavelengths just beyond the human visible spectrum, ranging from 100 to 400 nanometers. The Hubble Space Telescope has been our main instrument for observing UV light from space, including young stars forming in Spiral Galaxy NGC 3627, the auroras of Jupiter, and a giant cloud of hydrogen evaporating from an exoplanet that is reacting to its star’s extreme radiation.

Our sun and other stars emit a full range of UV light, telling astronomers how relatively hot or cool they are according to the subdivisions of ultraviolet radiation: near ultraviolet, middle ultraviolet, far ultraviolet, and extreme ultraviolet. Applying a false-color visible light composite lets us see with our own eyes the differences in a star’s gas temperatures.

Hubble’s Wide Field Camera 3 (WFC3) breaks down ultraviolet light into specific present colors with filters. “Science visuals developers assign primary colors and reconstruct the data into a picture our eyes can clearly identify,” according to the Hubble website . Using image-processing software, astronomers and even amateur enthusiasts can turn the UV data into images that are not only beautiful, but also informative.

X-Ray Light

chandra xray telescope image of two galaxies colliding and forming a gas bridge between them

Since 1999, the orbiting Chandra X-Ray Observatory is the most sensitive radio telescope ever built. During one observation that lasted a few hours, its X-ray vision saw only four photons from a galaxy 240 million light-years away, but it was enough to ascertain a novel type of exploding star . The observatory, located 86,500 miles above Earth, can produce detailed, full-color images of hot X-ray-emitting objects, like supernovas, clusters of galaxies and gases, and jets of energy surrounding black holes that are millions of degrees Celsius. It can also measure the intensity of an individual X-ray wavelength, which ranges from just 0.01 to 10 nanometers. Its four sensitive mirrors pick up energetic photons and then electronic detectors at the end of a 30-foot optical apparatus focus the beams of X-rays.

Closer to home, the Aurora Borealis at the poles emits X-rays too. And down on Earth, this high-frequency, low-wavelength light passes easily through the soft tissue of our bodies, but not our bones, yielding stellar X-ray images of our skeletons and teeth.

Visible Light

visible light image of mystic mountain, a pillar of gas dust and newborn stars in the carina nebula taken by the hubble telescope

Visible color gives astronomers essential clues to a whole world of information about a star, including temperature, distance, mass, and chemical composition. The Hubble Telescope, perched 340 miles above our planet, has been a major source of visible light images of the cosmos since 1990.

Hotter objects, like young stars, radiate energy at shorter wavelengths of light; that’s why younger stars at temperatures up to 12,000 degrees Celsius, like the star Rigel, look blue to us. Astronomers can also tell the mass of a star from its color. Because mass corresponds to temperature, observers know that hot blue stars are at least three times the mass of the sun. For instance, the extremely hot, luminous blue variable star Eta Carina’s bulk is 150 times the mass of our sun, and it radiates 1,000,000 times our sun’s energy.

Our comparatively older, dimmer sun is about 5,500 degrees Celsius, so it appears yellow. At the other end of the scale, the old star Betelgeuse has been blowing off its outer layer for the past few years, and it looks red because it’s only about 3,000 degrees Celsius.

A View of Earth

space telescopes and what lightwave ranges they detect

Scientists use different wavelengths of light to study phenomena closer to home, too.

Detectors in orbit can distinguish between geophysical and environmental features on Earth’s changing surface, such as volcanic action. For example, infrared light used alongside visible light detection reveals areas covered in snow, volcanic ash, and vegetation. The Moderate Resolution Imaging Spectroradiometer ( MODIS ) infrared instrument onboard the Aqua and Terra satellites monitors forest fire smoke and locates the source of a fire so humans don’t have to fly through smoke to evaluate the situation.

Next year, a satellite will be launched to gauge forest biomass using a special radar wavelength of about 70 centimeters that can penetrate the leafy canopy.

💡 Why is the sky blue? During the day, oxygen and nitrogen in Earth’s atmosphere scatters electromagnetic energy at the wavelengths of blue light (450–485 nanometers). At sunset, the sun’s light makes a longer journey through the atmosphere before greeting your eyes. Along the way, more of the sun’s light is scattered out of the blue spectrum and deeper into yellow and red.

Headshot of Manasee Wagh

Before joining Popular Mechanics , Manasee Wagh worked as a newspaper reporter, a science journalist, a tech writer, and a computer engineer. She’s always looking for ways to combine the three greatest joys in her life: science, travel, and food.

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September 22, 2011

Particles Found to Travel Faster Than Speed of Light

Neutrino results challenge a cornerstone of Albert Einstein's special theory of relativity, which itself forms the foundation of modern physics

By Geoff Brumfiel & Nature magazine

An Italian experiment has unveiled evidence that fundamental particles known as neutrinos can travel faster than light. Other researchers are cautious about the result, but if it stands further scrutiny, the finding would overturn the most fundamental rule of modern physics—that nothing travels faster than 299,792,458 meters per second.

The experiment is called OPERA (Oscillation Project with Emulsion-tRacking Apparatus), and lies 1,400 meters underground in the Gran Sasso National Laboratory in Italy. It is designed to study a beam of neutrinos coming from CERN, Europe's premier high-energy physics laboratory located 730 kilometers away near Geneva, Switzerland. Neutrinos are fundamental particles that are electrically neutral, rarely interact with other matter, and have a vanishingly small mass. But they are all around us—the sun produces so many neutrinos as a by-product of nuclear reactions that many billions pass through your eye every second. [ Click here to read more about CERN's Large Hadron Collider ]

The 1,800-tonne OPERA detector is a complex array of electronics and photographic emulsion plates, but the new result is simple—the neutrinos are arriving 60 nanoseconds faster than the speed of light allows. "We are shocked," says Antonio Ereditato, a physicist at the University of Bern in Switzerland and OPERA's spokesman.

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Breaking the law

The idea that nothing can travel faster than light in a vacuum is the cornerstone of Albert Einstein's special theory of relativity, which itself forms the foundation of modern physics. If neutrinos are traveling faster than light speed , then one of the most fundamental assumptions of science—that the rules of physics are the same for all observers—would be invalidated. "If it's true, then it's truly extraordinary," says John Ellis, a theoretical physicist at CERN.

Ereditato says that he is confident enough in the new result to make it public. The researchers claim to have measured the 730-kilometer trip between CERN and its detector to within 20 centimeters. They can measure the time of the trip to within 10 nanoseconds, and they have seen the effect in more than 16,000 events measured over the past two years. Given all this, they believe the result has a significance of six-sigma—the physicists' way of saying it is certainly correct. The group will present their results September 23 at CERN, and a preprint of their results will be posted on the physics website ArXiv.org .

At least one other experiment has seen a similar effect before, albeit with a much lower confidence level. In 2007, the Main Injector Neutrino Oscillation Search (MINOS) experiment in Minnesota saw neutrinos from the particle-physics facility Fermilab in Illinois arriving slightly ahead of schedule. At the time, the MINOS team downplayed the result, in part because there was too much uncertainty in the detector's exact position to be sure of its significance, says Jenny Thomas, a spokeswoman for the experiment. Thomas says that MINOS was already planning more accurate follow-up experiments before the latest OPERA result. "I'm hoping that we could get that going and make a measurement in a year or two," she says.

Reasonable doubt

If MINOS were to confirm OPERA's find, the consequences would be enormous. "If you give up the speed of light, then the construction of special relativity falls down," says Antonino Zichichi, a theoretical physicist and emeritus professor at the University of Bologna, Italy. Zichichi speculates that the "superluminal" neutrinos detected by OPERA could be slipping through extra dimensions in space, as predicted by theories such as string theory.

Ellis, however, remains skeptical. Many experiments have looked for particles traveling faster than light speed in the past and have come up empty-handed, he says. Most troubling for OPERA is a separate analysis of a pulse of neutrinos from a nearby supernova known as 1987a. If the speeds seen by OPERA were achievable by all neutrinos, then the pulse from the supernova would have shown up years earlier than the exploding star's flash of light; instead, they arrived within hours of each other. "It's difficult to reconcile with what OPERA is seeing," Ellis says.

Ereditato says that he welcomes skepticism from outsiders, but adds that the researchers have been unable to find any other explanation for their remarkable result. "Whenever you are in these conditions, then you have to go to the community," he says.

This article is reproduced with permission from the magazine Nature . The article was first published on September 22, 2011.

StickMan Physics

StickMan Physics

Animated Physics Lessons

Refraction of Light

Learn about the refraction of light and Snell's law. Learn how to solve for the angle that light bends when going from one medium to another.

The reason why a pencil in water appears to be bending is due to the speed of light in different medium.  Light bends when approaching a boundary between to medium where the speed of light is different.  Light travels faster in air and slower in water creating the phenomenon you see here.

Refraction of Light Variables

The speed of light in various medium.

Light is an electromagnetic wave that travels fastest in the vacuum of space, slower in gas, and slowest in a solid because of density.

  • Review our other lesson about speed of an electromagnetic wave and density following this link.

Light has a different speed in different translucent solids.  Translucent means you can see through them.  The velocity that light will travel in a certain medium can be calculated using a variable called the index of refraction ( n ).  This will also be used later on this page to determine how much light will bend from one medium to another.

Speed of an Electromagnetic Wave

Index of Refraction (n)

The index of refraction is the ratio of speed of light in a vacuum ( c ) to speed of light in another medium ( v ).

  • The index of refraction (n) is always 1 or greater
  • The larger the index of refraction (n) the slower light travels in that medium
  • The index of refraction (n) has as no unit

The variable c is a constant that stands for the speed of light in a vacuum.  This is the universal speed limit and always 3.0 x 10 8 m/s with nothing faster.

The index of refraction equation is:

Variables in this equation are:

c = 3.0 x 10 8 m/s

n = index of refraction

v = velocity of light in a medium

Index of Refraction Equation

Since c is a constant 3.0 x 10 8 m/s, the index of refraction equation n = c/v will always be n = (3.0 x 10 8 )/v

The index of refraction (n) will never be less than 1 since the velocity of light in any medium (v) can not be faster than the universal speed limit (c) of 3.0 x 10 8 m/s.  The denominator of n =c/ v  will never be bigger than the numerator and therefore not give an index of refraction (n) below 1.

Example Problems

1. What is the speed of light in water that has an index of refraction of 1.333?

rearranges to v = c/n

v = (3.0 x 10 8 )/(1.333) =

225056264.1 m/s

rounded in scientific notation becomes

v = 2.25 x 10 8 m/s

2. What is the speed of light in diamond that has an index of refraction of 2.419?

v = (3.0 x 10 8 )/(2.419) =

124018189.3 m/s

v = 1.24 x 10 8 m/s

3. What is the index of refraction for ice at 20° C where light travels at 2.29 x 10 8 m/s?

v = 2.29 x 10 8 m/s

n = (3.0 x 10 8 )/(2.29 x 10 8 ) =

4. Does light travel faster in diamond with an index of refraction of 2.419 or cubic zirconia with an index of refraction of 2.20?

The larger the index of refraction (n) the slower light is traveling in the medium.

Refraction of Light Into New Medium

Light changing speed in a new medium creates a bending.

  • If light is traveling slower in the new medium it will bend towards the normal line
  • If light is traveling faster in the new medium it will bend away from the normal line

Slower Medium to Faster Medium

Observe that as light enters the right side of the diagram it has a smaller index of refraction (n r ) which means it is traveling faster.  Light bends away from the normal line on the second or refracted side when this happens.

  • Slower medium to faster medium
  • (n i ) bigger than (n r )
  • Bends away from the normal line

Refraction Slower Medium to Faster

Faster Medium to Slower Medium

Observe that as light enters the right side of the diagram it has a larger index of refraction (n r ) which means it is traveling slower.  Light bends towards the normal line on the second or refracted side when this happens.

  • Faster medium to slower medium
  • (n i ) smaller than (n r )
  • Bends toward the normal line

Refraction Faster to Slower

Snell's Law

Snell's law can be used to mathematically calculate the angle of bend or index of refraction on the incident side or refracted side when light travels between two medium.

With two medium we need subscripts to specify the side light is traveling in.  Use subscripts (i) for incident side and (r) for refracted side.

  • Incident (i) : first side where light starts
  • Refracted(r) : second side where light goes

(n i )(sin Ө i ) = (n r )(sin Ө r )

N i = index of refraction on the incident side, ө i = angle of refraction on the incident side, n r = index of refraction on the refracted side, ө r = angle of refraction on the refracted side.

Starting Snell's Law Problems

Calculation Tips With Snell's Law

You will have to do a bit of algebra to solve for various components in Snell's law.  Here are the rearranged equations you can check your work with.

  • When using Sin and Sin -1 you will have to make sure your calculator’s mode is in degrees .  With most TI graphing calculators there is a button for mode.  When you hit mode, there is an option for radian or degree.  Make sure it is on degrees.  This may be a reason if you do everything right but your calculated answer is wrong.
  • Be sure to use the correct trig function Sin or Sin -1 if you are not getting the right answer this may also be why.
  • Use lots of (parenthesis) , your calculator follows the order of operations and this makes sure the entire value you are calculating is in the right place

Rearranged Snell's Law Equation

5. Will light bend toward or away from the normal line when going from a slow medium to a fast medium?

Away form the normal line

6. Will light bend toward or away from the normal line when going from air (n=1.00) to water (n=1.33)?

Towards the normal line.

n i = smaller so faster

n r = bigger so slower

7. What is the angle of refraction when the angle of incidence is 20° when going from air (n=1.00) to water (n=1.33)?

Question 7

8. What is the angle of incidence when the angle of refraction is 15 ° when going from diamond (n=2.42) to water (n=1.33)?

Question 8

9. What is the index of refraction for an unknown substance when the angle of incidence is 20° in air (n=1.00) and angle of refraction is 15° in the unknown substance?

Question 9

10. What is the index of refraction for an unknown substance when the angle of incidence is 31 ° in the unknown and a angle of refraction is 26° in water (n=1.33) ?

Question 10

Total Internal Reflection

Light bends away from the normal line when going from a slower to a faster medium.  As the angle of incidence increases, so does the angle of refraction.  Light no longer refracts into a faster medium after a certain angle called the critical angle.  Beyond the critical angle you have total internal reflection and no refraction.

Solving for the critical angle (Ө c ):

Ө c = sin -1 ((n r )/ (n i ))

  • Total Internal Reflection : After passing the critical angle, light no longer refracts only reflects
  • Critical Angle : Angle of incidence after which light will only reflect and not refract
  • You will only have a critical angle and total internal reflection when light travels from slow to fast medium where light bends away from the normal line.

Example Problem

11. What is the critical angle for light in diamond (2.42) going into to air (1.00)?

Question 11

Click Here For the PhET Snell's Law Mini Lab Lab sheet And Walkthrough

  • On to Other EM Wave Phenomenon
  • Back to the  Main Electromagnetic Waves Page
  • Back to the  Stickman Physics Home Page
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Physics LibreTexts

5.1.1: Speeds of Different Types of Waves

  • Last updated
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  • Page ID 26167

  • Kyle Forinash and Wolfgang Christian

The speed of a wave is fixed by the type of wave and the physical properties of the medium in which it travels. An exception is electromagnetic waves which can travel through a vacuum. For most substances the material will vibrate obeying a Hooke's law force as a wave passes through it and the speed will not depend on frequency. Electromagnetic waves in a vacuum and waves traveling though a linear medium are termed linear waves and have constant speed. Examples:

  • For sound waves in a fluid (for example air or water) the speed is determined by \(v=(B/\rho )^{1/2}\) where \(B\) is the bulk modulus or compressibility of the fluid in newtons per meter squared and \(\rho\) is the density in kilograms per cubic meter.
  • For sound waves in a solid the speed is determined by \(v= (Y/\rho )^{1/2}\) where \(Y\) is Young's modulus or stiffness in Newtons per meter squared and \(\rho\) is the density in kilograms per meter cubed.
  • For waves on a string the speed is determined by \(v=(T/\mu )^{1/2}\) where \(T\) is the tension in the string in Newtons and \(\mu\) is the mass per length in kilograms per meter.
  • Although electromagnetic waves do not need a medium to travel (they can travel through a vacuum) their speed in a vacuum, \(c = (1/\mu _{o} ε_{o})^{1/2} = 3.0\times 10^{8}\text{ m/s}\) is governed by two physical constants, the permeability \(\mu_{o}\) and the permittivity, \(ε_{o}\) of free space (vacuum).

Table \(\PageIndex{1}\)

Here is a more comprehensive list of the speed of sound in various materials .

As we saw in the previous chapter, there is a relationship between the period, wavelength and speed of the wave. The period of a cork floating in the water is affected by how fast the wave passes (wave speed) and the distance between peaks (wavelength). The relationship between speed, period and wavelength of a sine wave is given by \(v=\lambda /T\) where wavelength and period for a sine wave were defined previously. This can also be written as \(v=\lambda f\) since frequency is the inverse of period and is true for all linear waves. Notice that, since wave speed is normally a fixed quantity the frequency and wavelength will be inversely proportion; higher frequencies mean shorter wavelengths.

Often it is easier to write \(ω = 2πf\) where \(\omega\) is the angular frequency in radians per second instead of having to write \(2\pi f\) everywhere. Likewise it is easier to write \(k=2\pi /\lambda \) where \(k\) is the wave number in radians per meter rather than having to write \(2\pi /\lambda\) a lot. (Note that \(k\) is not a spring constant here.) Using these new definitions the speed of a wave can also be written as \(v=f\lambda =\omega /k\).

If the medium is uniform the speed of a wave is fixed and does not change. There are circumstances where the speed of a particular wave does change, however. Notice that the speed of sound in air depends on the density of the air (mass per volume). But the density of air changes with temperature and humidity. So the speed of sound can be different on different days and in different locations. The temperature dependence of the speed of sound in air is given by \(v = 344 + 0.6 (T - 20)\) in meters per second where \(T\) is the temperature in Celsius (\(T\) here is temperature, not period). Notice that at room temperature (\(20^{\circ}\text{C}\)) sound travels at \(344\text{ m/s}\).

The speed of sound can also be affected by the movement of the medium in which it travels. For example, wind can carry sound waves further (i.e. faster) if the sound is traveling in the same direction or it can slow the sound down if the sound is traveling in a direction opposite to the wind direction.

Electromagnetic waves travel at \(\text{c} = 3.0\times 10^{8}\text{ m/s}\) in a vacuum but slow down when they pass through a medium (for example light passing from air to glass). This occurs because the material has a different value for the permittivity and/or permeability due to the interaction of the wave with the atoms of the material. The amount the speed changes is given by the index of refraction \(n=c/v\) where \(c\) is the speed of light in a vacuum and \(v\) is the speed in the medium. The frequency of the wave does not change when it slows down so, since \(v=\lambda f\), the wavelength of electromagnetic waves in a medium must be slightly smaller.

Video/audio examples:

  • What is the speed of sound in a vacuum? Buzzer in a bell jar . Why is there no sound when the air is removed from the jar?
  • Demonstration of speed of sound in different gasses . Why is there no sound when the air is removed from the jar?
  • These two videos demonstrate the Allasonic effect. The speed of sound is different in a liquid with air bubbles because the density is different. As the bubbles burst, the speed of sound changes, causing the frequency of sound waves in the liquid column to change, thus changing the pitch. Example: one , two . What do you hear in each case?
  • The Zube Tube is a toy that has a spring inside attached to two plastic cups on either end. Vibrations in the spring travel at different speeds so a sound starting at one end (for example a click when you shake the tube and the spring hits the cup) ends up changing pitch at the other end as the various frequencies arrive. In other words this is a nonlinear system. See if you can figure out from the video which frequencies travel faster, high frequencies or low.

Mini-lab on measuring the speed of sound .

Questions on Wave Speed:

\(f=1/T,\quad v=f\lambda ,\quad v=\omega /k,\quad k=2\pi /\lambda,\quad \omega =2\pi f,\quad y(x,t)=A\cos (kx-\omega t+\phi ),\quad v=\sqrt{B/Q}\)

  • Light travels at \(3.0\times 10^{8}\text{ m/s}\) but sound waves travel at about \(344\text{ m/s}\). What is the time delay for light and sound to arrive from a source that is \(10,000\text{ m}\) away (this can be used to get an approximate distance to a thunderstorm)?
  • What two mistakes are made in science fiction movies where you see and hear an explosion in space at the same time?
  • Consult the table for the speed of sound in various substances. If you have one ear in the water and one ear out while swimming in a lake and a bell is rung that is half way in the water some distance away, which ear hears the sound first?
  • At \(20\text{C}\) the speed of sound is \(344\text{ m/s}\). How far does sound travel in \(1\text{ s}\)? How far does sound travel in \(60\text{ s}\)?
  • Compare the last two answers with the distance traveled by light which has a speed of \(3.0\times 10^{8}\text{ m/s}\). Why do you see something happen before you hear it?
  • The speed of sound in water is \(1482\text{ m/s}\). How far does sound travel under water in \(1\text{ s}\)? How far does sound travel under water in \(60\text{ s}\)?
  • What happens to the speed of sound in air as temperature increases?
  • Using the equation for the speed of sound at different temperatures, what is the speed of sound on a hot day when the temperature is \(30^{\circ}\text{C}\)? Hint: \(v = 344\text{ m/s} + 0.6 (T - 20)\) where \(T\) is the temperature in Celsius.
  • Using the speed of sound at \(30^{\circ}\text{C}\) from the last question, recalculate the distance traveled for the cases in question four.
  • Suppose on a cold day the temperature is \(-10^{\circ}\text{C}\: (14^{\circ}\text{F}\)). You are playing in the marching band outside. How long does it take the sound from the band to reach the spectators if they are \(100\text{ m}\) away?
  • What is the difference in the speed of sound in air on a hot day (\(40^{\circ}\text{C}\)) and a cold day (\(0^{\circ}\text{C}\))?
  • What would an orchestra sound like if different instruments produced sounds that traveled at different speeds?
  • The speed of a wave is fixed by the medium it travels in so, for a given situation, is usually constant. What happens to the frequency of a wave if the wavelength is doubled?
  • What happens to the wavelength of a wave if the frequency is doubled and has the same speed?
  • Suppose a sound wave has a frequency \(200\text{ Hz}\). If the speed of sound is \(343\text{ m/s}\), what wavelength is this wave?
  • What factors determine the speed of sound in air?
  • Why do sound waves travel faster through liquids than air?
  • Why do sound waves travel faster through solids than liquids?
  • The speed of sound in a fluid is given by \(v=\sqrt{B/Q}\) where \(B\) is the Bulk Modulus (compressibility) and \(Q\) is the density. What happens to the speed if the density of the fluid increases?
  • What must be true about the compressibility, \(B\), of water versus air, given that sound travels faster in water and water is denser than air?
  • The speed of sound in a fluid is given by \(v=\sqrt{B/Q}\) where \(B\) is the Bulk Modulus (compressibility) and \(Q\) is the density. Can you think of a clever way to measure the Bulk Modulus of a fluid if you had an easy way to measure the speed of sound in a fluid? Explain.
  • The speed of sound on a string is given by \(v=\sqrt{T/\mu}\) where \(T\) is the tension in Newtons and \(\mu\) is the linear density (thickness) in \(\text{kg/m}\). You also know that \(v=f\lambda\). Give two ways of changing the frequency of vibration of a guitar string based on the knowledge of these two equations.
  • For the previous question, increasing the tension does what to the frequency? What does using a denser string do to the frequency?
  • The following graph is of a wave, frozen in time at \(t = 0\). The equation describing the wave is \(y(x,t)=A\cos (kx-\omega t+\phi )\). Sketch the effect of doubling the amplitude, \(A\).

clipboard_e5f764e637575a65b45b9f0eb09115dd2.png

Figure \(\PageIndex{1}\)

  • For the following graph of a wave, sketch the effect of doubling the wavelength.

clipboard_e37815ebeb535ef7dffe9296d845d3d06.png

Figure \(\PageIndex{2}\)

  • The mathematical description of a sine wave is given by \(y(x,t)=A\cos (kx-\omega t+\phi )\). Explain what each of the terms \((A, k, \omega, \phi )\) represent.

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Why does light travel faster in vacuum

Why does light travel faster in vacuum

Light is an incredible phenomenon that has puzzled humanity for centuries. It has always been a fascinating subject, and over the years, many questions have been asked about it.

One of such questions is why light travels faster in a vacuum than in any other medium.

Alpha Centauri is 4.3 light-years away. “To obtain an idea of the size of a light-year, take the circumference of the Earth (24,900 miles), lay it out in a straight line , multiply the length of the line by 7.5 (the corresponding distance is one light-second), then place 31.6 million similar lines end to end,” NASA’s Glenn Research Center says on its website.

Electromagnetic wave

They are waves created as a result of vibrations of the electric and magnetic fields. i.e., they are oscillating electric and magnetic fields. Radio waves , microwaves, visible lights, and x-rays are all examples of electromagnetic waves.

We will delve into the physics behind the movement of light, explaining how light moves, why it travels faster in a vacuum and what it means for us.

Light is one of the most fundamental components of the universe. It is the fastest thing in the universe, and it is the basis for all forms of electromagnetic radiation.

However, it is common knowledge that light travels faster in a vacuum than in any other medium. This raises the question: why does light travel faster in a vacuum?

The answer lies in the fact that light is an electromagnetic wave.

Electromagnetic waves are created by the interaction of electric and magnetic fields.

When light travels through a medium, it interacts with the atoms and molecules in that medium, which slows it down.

In a vacuum, there are no atoms or molecules to interact with, so light can travel at its maximum speed of 299,792,458 meters per second.

This same speed is known as the speed of light, and it is the fastest speed that anything can travel in the universe.

What is Light?

Light is an electromagnetic wave that has properties of both waves and particles. It is a form of energy that travels through space at a constant speed of 299,792,458 meters per second. Light is a type of radiation that is visible to the human eye and is responsible for enabling us to see the world around us.

  • Light travels through transparent mediums at a finite speed, and the speed depends on the refractive index of the medium.
  • When light passes through a rotating mirror, it changes direction in such a way that its speed remains constant.
  • In a vacuum, where there is no material to slow it down, light travels faster and maintains a constant rate.
  • According to Einstein’s special theory of relativity, the speed of light is the same for all observers regardless of their reference frame or different speeds.
  • This means that even if a moving object is traveling faster than the speed of light, it will still be unable to travel faster than the speed limit imposed by the laws of physics.
  • Danish astronomer Ole Rømer first demonstrated the finite speed of light in the 17th century by observing the time difference in the arrival of light from Jupiter’s moon depending on the position of the Earth in its orbit.

Wave or Particle?

The nature of light has been a topic of debate for many years. On one hand, light exhibits wave-like behavior, such as diffraction and interference. On the other hand, it also exhibits particle-like behavior, such as the photoelectric effect and Compton scattering. Scientists have come to the conclusion that light behaves as both a wave and a particle, depending on the experiment being conducted.

Wave-like behavior

When light travels through a medium, it behaves like a wave. This is because light waves can be diffracted and refracted, which means they can bend and change direction as they pass through different materials. This is why light appears to bend when it passes through a prism or a glass of water.

Particle-like behavior

When light interacts with matter, it behaves like a particle. This is because light is made up of discrete packets of energy called photons. These photons can be absorbed or emitted by matter, causing electrons to be excited or ionized. This is why light can cause chemical reactions and produce electrical currents.

In conclusion, light is a form of energy that behaves as both a wave and a particle. Its properties are determined by the experiment being conducted and the medium it is traveling through.

How does light travel?

Through a medium.

When light travels through a medium, such as air, water, or glass, it interacts with the atoms and molecules in the medium. This interaction causes the light to slow down, and the amount of slowing depends on the properties of the medium. The slowing of light in a medium is described by the medium’s refractive index, which is a measure of how much the speed of light is reduced in the medium.

In a Vacuum

In a vacuum, there are no atoms or molecules for the light to interact with. As a result, light can travel through a vacuum at its maximum possible speed, which is approximately 299,792,458 meters per second (or about 186,282 miles per second). This speed is known as the speed of light in a vacuum, and it is a fundamental constant of the universe.

The fact that light can travel through a vacuum at its maximum possible speed is one of the most important discoveries in physics. It means that light can travel enormous distances through space without being slowed down by any intervening matter. This is why we are able to see stars and galaxies that are billions of light-years away from us.

In addition to being able to travel through a vacuum, light also behaves as both a wave and a particle. This duality is known as wave-particle duality, and it is one of the most fascinating aspects of quantum mechanics.

Overall, the speed of light in a vacuum is a fundamental constant of the universe, and it plays a crucial role in our understanding of the cosmos.

Why does light travel faster in vacuum?

Special relativity.

According to special relativity, the speed of light in vacuum is the same for all observers, regardless of their relative motion. This means that the speed of light is a fundamental constant of the universe. The theory of special relativity also predicts that as an object approaches the speed of light, its mass increases and its length contracts. However, light itself has no rest mass, so it is not subject to these effects.

Mass and Energy

In physics, mass and energy are equivalent, as described by Einstein’s famous equation E=mc². This means that the energy of a photon is directly proportional to its frequency, and inversely proportional to its wavelength. In a vacuum, there are no particles to interact with the photons, so they can travel unimpeded and at their maximum speed.

Electromagnetic Waves

Light is an electromagnetic wave, consisting of oscillating electric and magnetic fields. These electric and magnetic fields can propagate through a vacuum because they do not require a medium to travel through. In other words, the electromagnetic waves charged particles that make up light can exist independently of matter.

In summary, the reason why light travels faster in vacuum is due to a combination of special relativity, the equivalence of mass and energy free space, and the nature of electromagnetic waves. These factors allow light and electromagnetic waves to travel unimpeded and at its maximum speed through empty space.

Applications of Light in Vacuum

Communication.

Light in vacuum is used extensively in modern communication systems, such as fiber optic cables. These cables are made up of thin strands of glass or plastic that transmit light signals over long distances with minimal loss of signal strength. This technology has revolutionized the telecommunications industry, allowing for faster and more reliable communication over long distances.

Light in vacuum is also essential in the field of astronomy. Astronomers use telescopes equipped with special sensors that can detect different wavelengths of light. By studying the light emitted by stars and galaxies, astronomers can learn about their composition, temperature, and distance from Earth. This information helps us to better understand the universe and our place in it.

In addition, the study of light in vacuum has led to the development of new technologies, such as lasers and LED lights. These technologies have a wide range of applications, from medical treatments to industrial manufacturing.

Overall, the study of light in vacuum has had a profound impact on modern science and technology in distant universe. By understanding the properties of light in vacuum, we have been able to develop new technologies and gain a deeper understanding of the universe around us.

However, in a vacuum, there are no particles to interact with electromagnetic wave, so the electromagnetic fields of light can propagate unimpeded, allowing it to travel at its maximum speed of approximately 299,792,458 meters per second.

The speed of light in a vacuum is a fundamental constant of the universe and is used as a reference for measuring the speed of other objects. It is also the basis for many important scientific theories, such as Einstein’s theory of relativity.

Understanding why light travels faster in a vacuum is essential for many fields of science, including astronomy, optics, and quantum mechanics. It is also critical for the development of advanced technologies, such as fiber-optic communication systems and laser technology.

1. The Speed of Light: Understanding its Significance

The speed of light, known as “c,” is a universal constant that travels at constant rate of exactly 299792458 meters per second through a vacuum. This speed limit is a key concept in physics and helps define international standard measurements like the meter. Despite its reputation as a universal constant, scientists continue to explore the possibility of faster-than-light travel. [1] [2]

Why does light travel faster in vacuum 2. The Importance of a Vacuum in Understanding Light Speed

2. The Importance of a Vacuum in Understanding Light Speed

Understanding the speed of light in a vacuum is essential as it sets the universal constant for light. It serves as a speed limit to the universe, and nothing can surpass this limit. The vacuum speed of light remains constant no matter who measures it. [3] [4]

Why does light travel faster in vacuum 3. Index of Refraction: Definition and Applications

Source: pressbooks.bccampus.ca

3. Index of Refraction: Definition and Applications

The index of refraction is defined as the ratio of the speed of light in vacuum to the speed of light traveling through a material. It is used to describe how much light is slowed down when passing through a substance. The values of n can vary depending on the material, and those with large indices are optically dense while those closer to one are optically rare. [5] [6]

Light traveling through anything other than a perfect vacuum will scatter off off whatever particles exist

Why does light travel faster in vacuum 4. Optically Dense vs. Optically Rare Materials

Source: www.rpi.edu

4. Optically Dense vs. Optically Rare Materials

Optically dense materials slow down the speed of light passing through them due to their higher refractive index. In contrast, optically rare materials allow light to pass through faster. This difference different speeds is due to how much energy the medium’s particles can absorb and vibrate before emitting an electromagnetic wave. [7] [8]

Why does light travel faster in vacuum 5. Difference between Air and Vacuum in Index of Refraction

5. Difference between Air and Vacuum in Index of Refraction

In general relativity in terms of index of refraction, air and vacuum are very similar, with the index of refraction of air being very close to 1.0. However, in vacuum, the speed of light is always constant and equal to c=2.99792458 x 108 m/s, regardless of any external factors. [9] [10]

Why does light travel faster in vacuum 6. Light in Matter and Slowing Down

6. Light in Matter and Slowing Down

When light travels through a non-vacuum, like water or glass, it slows down from its constant speed of c. This is why we see images bend when it goes through water, and why glasses and binoculars work. The speed of light instantly changes back once it exits the medium. The refractive index determines the actual speed of light in different media, and it is not due to light particles just traveling faster along a longer path or getting absorbed and re-emitted by atoms. [11] [12]

Why does light travel faster in vacuum 7. Maximum Speed Limit in the Universe: The Speed of Light

7. Maximum Speed Limit in the Universe: The Speed of Light

The speed of light through a vacuum is a universal constant known as c, and is the speed limit in the universe. According to Einstein’s theory of special relativity, nothing can travel faster than light. The speed of light is used to define international standard measurements such as the meter and kilogram. [13] [14]

Why does light travel faster in vacuum 8. Travelling Faster than Light: Fiction or Reality?

8. Travelling Faster than Light: Fiction or Reality?

Scientists and science fiction writers spend time contemplating faster-than-light travel, but so far no one has been able to demonstrate a real warp drive. While some theories propose apparent faster-than-light travel, the current scientific consensus is that nothing may travel faster than the speed of light. [15] [16]

Why does light travel faster in vacuum 9. Measuring Distance in the Universe using Light-Years

9. Measuring Distance in the Universe using Light-Years

Light-years are a common way to measure the vast distances in the universe. It’s the distance light travels in one year – about 6 trillion miles. Astronomers use this measurement to study objects beyond our solar system, some of which are billions of light-years away, allowing them to see the universe as it looked billions of years ago. [17] [18]

Why does light travel faster in vacuum 10. History of Discovering the Speed of Light.

10. History of Discovering the Speed of Light.

The history of discovering the speed of light dates back to ancient Greek philosophers like Empedocles and Aristotle who debated over its nature. Ole Roemer first demonstrated its finite speed and James Clerk Maxwell proposed that light was an electromagnetic wave. Einstein’s theory of relativity showed that the speed of light is a constant and is independent of the motion of the light source. [19] [20]

Why does light travel faster in vacuum Can anything travel faster than the speed of light?

Source: bigthink.com

Can anything travel faster than the speed of light?

According to Albert Einstein’s theory of special relativity, nothing in the universe can travel faster than the speed of light. This theory has been supported by various experiments, and the speed of light is considered a speed limit on everything in the universe. Therefore, nothing can travel faster than the speed of light. [21] [22]

Why does light travel faster in vacuum How Fast Is the Speed of Light in Air and Water?

How Fast Is the Speed of Light in Air and Water?

The speed of light in air is slightly lower and slower than its speed in a vacuum, while it is even slower in transparent medium of water due to its higher refractive index. However, no matter what medium light travels through, its speed is always constant and limited by the ultimate speed limit of the universe, which is the speed of light in a vacuum. [23] [24]

When light entering an alternative medium like water or glasses in the air travels slower (see figure 2) but its frequency remains unchanged. Light travels at same speed of about 30000 kilometers per second in a vacuum with a refractive index, and it slows to 225,000 kilometers per minute in water and 210,000 km per second in air. In diamond, the speed of a light accelerated at one medium at relative speeds of 125,000 km per second is about 61 percent below a normal speed in the absence of a light.

Why does light travel faster in vacuum Fizeau and Foucault's Speed of Light Experiments

Fizeau and Foucault\’s Speed of Light Experiments

Fizeau and Foucault used rotating mirrors in the mid-1800s to measure the speed of light in different media. Foucault improved upon Fizeau’s method and also proved that light traveled slower in water than in air. Their experiments led to a more accurate determination of the speed of light. [25] [26]

While in France, rival researchers Armand Fizeau and Jean-Bernard-Leon Foucault tried to measure light speed independently using Arago’s findings as well as expanding Wheatstone’s results. In 1848 Fizeau developed a device where a rotating mirror reflects a light through the toothed wheel and then into an angled mirror. He was able in an instant to rotate the wheel in two teeth to catch light from the nearby gap while returning from an external journey.

Why does light travel faster in vacuum Can we travel faster than light?

Can we travel faster than light?

According to Albert Einstein’s theory of special relativity, nothing in the universe can travel faster than light. Despite this, scientists and science fiction writers still contemplate the possibility of faster-than-light travel, but no real warp drive has been demonstrated yet. [27] [28]

Sci-fi adores ‘warp’ speeds. Faster-than-light travel allows for many sci-fi movies to be developed and expand space, while letting characters move easily between stars. Fast travel is certainly not impossible, but a lot more exotic theory would have been required to do it. Fortunately for sci-fi enthusiasts as well theoretical scientists, the possibilities for exploration can be huge. It just takes some time and effort to figure out such a way solution to our problems. Simple, okay.

Ole Roemer’s Speed of Light Estimations

In 1676, Ole Roemer became the first person to measure the speed of light while observing the orbit of Io, one of Jupiter’s satellites. He noticed that the time interval between eclipses of Io by Jupiter was affected by the relative positions of Earth and Jupiter. By calculating the time difference, he estimated that light required 22 minutes to cross the diameter of Earth’s orbit, leading to a speed of light equivalent to 131,000 miles per second. [29] [30]

Shortly after launching and introducing the telescope, some very crude improvements were done by Danish scientist Ole Roemer in light year 1676, the first astronomer to attempt to calculate light’s velocity. The study reveals that he predicts the periodicity of an eclipse period on Jupiter. After some months though, his forecasts gradually began getting less exact over progressively longer periods, reaching an error maximum of approximately 23 minutes.

Why does light travel faster in vacuum Einstein's Special Theory of Relativity and the Speed of Light

Einstein’s Special Theory of Relativity and the Speed of Light

Einstein’s Special Theory of Relativity states that the speed of light is constant in a vacuum, and no object can travel faster than light. However, when traveling through a material other than a vacuum, such as water or glass, light can slow down due to interactions with molecules. This slowing is described by the index of refraction, with materials having higher indices being considered optically dense. [31] [32]

In 1905 Einstein published his Special Theory of Relativity. In 1915 Einstein published his General Theory of Relativity. During their study, the second theory dealt with acceleration and its relationship with gravity, and the second theory concerned gravity and a static speed. Because the theory challenged many long awaited theories like Isaac Newton’s laws, Einsteins theory was a revolutionary force for physics. The concept of relativity embodies a concept that based on observations, the speed is determined only by observing the moving object itself.

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Atom Particles

Does light travel faster in glass or vacuum.

Table of Contents

Explain that unlike sound, light waves travel fastest through a vacuum and air, and slower through other materials such as glass or water.

Does light travel faster in a vacuum?

Light travels faster in vacuum than any other medium. This is because there is no obstruction in vacuum for the propagation of light and thus, the refractive index of vacuum is the lowest. Show the speed of light in vacuum is the maximum.

Why does light travel faster in glass?

When light goes through glass, it gets knocked around and bumps into all sorts of molecules and electrons. So whenever it’s traveling, it’s traveling at the speed of light. But it’s busy interacting with and scattering off lots of stuff along the way, and it doesn’t necessarily take the shortest path through the glass.

What travels faster than light in a vacuum?

“Nothing can travel faster than the speed of light in a vacuum.” “Light in a vacuum always travels at the same speed.” Those additional three words in a vacuum are very important. A vacuum is a region with no matter in it.

Why does light travel slower through glass than through a vacuum?

Light moves slower through denser media because more particles get in its way. Each time the light bumps into a particle of the medium, the light gets absorbed which causes the particle to vibrate a little and then the light gets re-emitted.

Does light slow down in a vacuum?

This finding shows unambiguously that the propagation of light can be slowed below the commonly accepted figure of 299,792,458 metres per second, even when travelling in air or vacuum.

Why does light slow down when not in a vacuum?

Light traveling through anything other than a perfect vacuum will scatter off of whatever particles exist. This scattering slows light down when light travels through any medium other than the empty space of a vacuum, as illustrated below.

Why can’t anything travel faster than light in a vacuum?

Hence, an object moving at the speed of light through space experiences no time at all or in other words is frozen in time. So, the real reason why we can’t move faster than the speed of light is that once we’re moving entirely through space, there’s no more speed to be gained.

Why is speed of light same in vacuum?

A vacuum, by definition, has no electric charge at all for the light to interact with. Therefore, its refractive does not depend on frequency. This last statement is true because (1) light is mediated by a massless field and (2) special relativity requires that all such fields propagate at exactly c.

Does light speed up in glass?

Refractive Index Light’s speed is reduced when it travels through a medium due to the interaction of photons with electrons. Typically, higher electron densities in a material result in lower velocities. This is why light travels fast in glass, faster in water, and fastest in a vacuum.

Why does light travel slowest in glass?

The more optically denser the medium is, the slower light propagates through it. Among the given media, glass is optically the densest, and has a typical refractive index of around 1.5.

What makes light the fastest?

Nothing can travel faster than 300,000 kilometers per second (186,000 miles per second). Only massless particles, including photons, which make up light, can travel at that speed. It’s impossible to accelerate any material object up to the speed of light because it would take an infinite amount of energy to do so.

What travels fastest in light?

Light has the maximum velocity in vacuum. Light shows the maximum velocity in a vacuum. So there is a vacuum in the options, you can be sure that it is the medium through which light travels faster. Air also has a refractive index very close to one (n=1.003) but not less than vacuum.

Which light travels fastest?

Red light having maximum wavelength travels fastest.

How fast does light move not in a vacuum?

Light from a stationary source travels at 300,000 km/sec (186,000 miles/sec).

In which light travels fastest?

Light travels the fastest in Air.

Does light move faster in glass than air?

The refractive index of glass is higher than air, so the speed of light will be slower in glass and faster in air.

Is the speed of light faster in glass?

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light travel fastest in vacuum

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Why does light travel faster in vacuum?

Light travels faster in vacuum than any other medium. this is because there is no obstruction in vacuum for the propagation of light and thus, the refractive index of vacuum is the lowest. show the speed of light in vacuum is the maximum..

Why can light travel in vacuum ?

Does sound travel faster than light?

Cleaning Beasts

Why does light travel faster in a vacuum, why does light travel faster in a vacuum with special relativity.

The rapidity of light is a speed limit on the whole thing in our universe. Light itinerant through a vacuum transfers at accurately 299,792,458 meters (983,571,056 feet) per second. That’s around 186,282 miles per second a general constant known in calculations and in shorthand as “c,” or the speed of light.

Agreeing to physicist Albert Einstein’s philosophy of special relativity, why does light travel faster in a vacuum , on which abundant of modern physics is established, a nonentity in the universe can collapsible faster than light? The philosophy states that as matter slants the speed of light, that matter’s mass becomes immeasurable. That means the rapidity of light functions as a speed limit on the entire universe. The speed of light is so absolute that, according to the U.S. National Organization of Standards and Technology, it is utilized to define international standard dimensions like the meter (and by leeway, the mile, the base, and the inch). Over some crafty equations, it also assists define the kilogram and the Kelvin.

But notwithstanding the speed of light’s status as a universal constant, experts and science literature writers alike spend time anticipating faster-than-light travel. So far away, no one’s been talented to figure out how to portable at such speed. But that hasn’t reduced our collective hurtle toward the latest stories, new creations, and new realms of physics.

What is Light-Year?

The Hubble Cosmos Telescope snapped this appearance of the twisting galaxy NGC 3972. The galaxy is 65 million light-years from the Ground (that’s 382 quintillion miles!) and can be established in the group Ursa Major.

The Hubble Space Shrivel snapped this image of the twisting galaxy NGC 3972. The galaxy is 65 million light-years from World (that’s 382 quintillion miles!) and can be initiated in the gathering, Ursa Major.

A light-year is a coldness that light can pass in one year about six trillion miles (10 trillion kilometers). It’s a single way that astronomers and physicists extent enormous distances across our universe.

Light movement from the moon to our eyes in around 1 second means the moon is about 1 light second far away. Daylight takes about 8 minutes to touch our eyes, so the sun is nearby 8 light minutes away. Bright from Alpha Centauri, which is the nearest star scheme to our own, needs roughly 4.3 years to acquire here, so Alpha Centauri is 4.3 light-years away.

“To get an idea of the size of a light-year, get the circumference of the Earth (24,900 miles), amateur it out in a conventional line, reproduce the length of the line by 7.5 (the conforming distance is one light-second), at that time place 31.6 million analogous lines end to end,” said NASA’s Glenn Investigation Center. “The resulting distance is virtually 6 trillion (6,000,000,000,000) miles!”

Stars and other substances beyond our solar system lie wherever from a few light-years to a limited billion light-years away. And the whole lot astronomers “see” in the aloof universe is factual history. When stargazers study substances that are far away, the objects seem as they happened at the time that light left them.

This belief allows astronomers to understand the universe as it looked afterward the Big Bang that took place nearly 13.8 billion years ago. Substances that are 10 billion light-years away seem to astronomers as they observed 10 billion years ago relatively soon afterward the beginning of the world rather than in what way they appear today.

How did we Learn the Speed of Light?

Galileo Galilei is accredited with discovering the primary four moons of Jupiter.

Aristotle, Empedocles, Galileo (exemplified here), Ole Rømer, and uncountable other philosophers and physicists in past have contemplated the rapidity of light.

As timely as the 5th century, Greek theorists like Empedocles and Aristotle distressed the nature of light speed. Empedocles supposed that light, whatever it was prepared of, must travel and consequently, must have a rate of go through. Aristotle transcribed a rebuttal of Empedocles’ opinion in his own treatise, On Sense and the Workable, arguing that light, different sound, and smell, are prompt. Aristotle was incorrect, of course, but it would take hundreds of years for anybody to prove it.

In the mid-1600s, checks PBS NOVA, the Italian astronomer Galileo Galilei erected two people on a hill less than a mile separately. Individual person held a protected lantern. One exposed his lantern; once the other person saw the vulgar, he naked his too. But Galileo’s untried distance wasn’t far sufficient for his participants to record the rapidity of light. He might only conclude that light moved at least 10 times faster than sound.

In the 1670s, Danish star watcher Ole Rømer strained to create a reliable schedule for sailors at sea, and rendering to NASA accidentally came up with the latest best evaluation for the speed of light. To make an astronomical timepiece, he logged the precise timing of conceals of Jupiter’s moon, Io, since Earth. Over the period, Rømer pragmatic that Io’s eclipses often varied from his controls. He observed that the eclipses seemed to lag the most when Jupiter and Earth were touching away from one additional, showed up ahead of a period when the planets were impending and occurred on plan when the planets were at their neighboring or farthest points, a rough form of the Doppler influence or redshift. In a leap of instinct, he determined that light was taking quantifiable time to travel from Io to Earth.

Rømer utilized his observations to estimate the speed of light . Meanwhile, the size of the solar scheme and Earth’s orbit wasn’t yet precisely known, contended a 1998 paper in the American Journal of Physics, he was a while off. But in past, scientists had an amount to work with. Romer’s intention put the speed of light at around 124,000 miles per second (200,000 km/s).

In 1728, English physicist James Bradley founded a new set of cunnings on the change in the seeming position of the leads due to Earth’s travels around the sun. He assessed the rapidity of light at 185,000 miles per second (301,000 km/s) correct to within about 1% of the actual value, according to the American Corporeal Society.

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light travel fastest in vacuum

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The 5 best cordless vacuum cleaners for 2024

Ditch the heavy machines and get a lightweight, cordless stick vacuum instead..

The vacuum world has gone cordless, untethering us from outlets and saving us fights with pesky wires. Cordless vacuums can be great alternatives to traditional corded models since they’re lightweight and easier to maneuver around your home. They’ve also gotten much more powerful in recent years. Some even come with extra features like app connectivity, self-emptying bases and more. But just like the robot vacuum space, there are dozens of cordless vacuums to choose from now (no, Dyson isn’t your only option). Before you upgrade to a cordless vacuum it’s important to do your research. We at Engadget can help make your decision a bit easier. After testing a bunch of the most popular cordless vacuum cleaners available today, we’ve come up with our top picks, plus loads of buying advice to help you figure out which cordless stick vacuum is right for you.

Dyson V15 Detect

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Most of the cordless vacuums you’ll find today have stick designs, with a handle at the top attached to a debris bin, which has a space to connect different attachments at one end. These designs are more versatile than old school vacuums of yesteryear because, while you may use the long stick attachment most of the time to clean your floors, many cordless vacuums come with other attachments as well. Some allow you to clean hard to reach spaces like the interior of your car, while others make it easier to vacuum furniture and clean inside crevices.

Bin volume is worth keeping in mind when you’re choosing a cordless vacuum. The larger the bin, the more debris it can hold, but it might also mean a heavier machine. All of the cordless vacuums we tested had a bin size between 0.1 and 0.8 gallons and all were able to handle cleaning an entire one-pet home (roughly 2,000 square feet) in a single run without needing to be emptied.

Some cordless vacuums also have removable, replaceable battery packs, which is super handy. That means you can buy a replacement battery and install it easily, without needing to seek out professional assistance. Also, these extra batteries cost around $150-$200 a pop — expensive, yes, but nowhere near as costly as buying a whole new cordless vacuum.

Suction and power modes

Cordless vacuum suction power is typically measured in air wattage (AW), but you’ll see some that list the power of the motor in wattage (W) instead. Typically the higher the air or motor wattage the stronger the suction power, and often strength is proportional to price — more expensive cordless vacuums tend to have stronger suction power. A general rule of thumb is that those with precocious pets or mess-making children would benefit from a cordless vacuum cleaner with stronger-than-average suction power.

Most cordless stick vacuums will have two different power modes: a “normal” or default power mode that balances suction power with battery life, as well as a “max” or stronger mode that kicks suction strength up a notch. Some vacuums, like those from Dyson, also have an “eco” mode, or one that prioritizes run time over strength.

Separate from power modes you can select yourself, some cordless vacuums will automatically adjust motor strength depending on the detected floor type or the amount of mess in its wake. Not only is this convenient, but it also ensures that the machine is working its hardest only when you need it.

Battery life

Obviously, battery life is important since you’ll probably want to clean more than one room in a shot. All of the cordless vacuums we tested had a battery life of at least 40 minutes in standard cleaning mode. I tested each by cleaning all three floors of my home (upstairs, downstairs and basement) on a single charge with the machine running in its standard (“auto”) mode and none of them ran out of juice before I could finish the third floor. That said, extra battery life can come in handy if you’re switching between power modes since “max” or high-power programs use more energy.

Most cordless stick vacuums come with some sort of base or mount where the machine lives when you’re not using it. Wall mounts are the most common, but some have free-standing bases where you dock and charge the vacuum. Consider the space in your home where you want the cordless vacuum to live, since it will have to have an outlet or another power source nearby.

Some high-end cordless vacuums come with self-emptying bases that act much like those included with expensive robot vacuums. After cleaning and returning the vacuum to the base, it will automatically empty the dustbin into a larger dustbin that you can then detach from the base when you need to empty it. This kind of base adds an extra layer of convenience into the mix, since you typically will only have to empty the larger dustbin every month or two.

“Smart” features

Yes, some cordless vacuums have “smart” features like Wi-Fi and app connectivity. But before we get into those, let’s talk about the extra perks scattered among these devices. Some models, like the latest from Dyson, include particle sensors that show you how many different sized pieces of debris it’s sucking up in real time. Dyson’s, for example, is a piezo acoustic sensor that detects particle size and frequency and displays that information on the vac’s LCD screen. Tineco’s iLoop sensor is similar, controlling its vacuums’ automatic suction power adjustment and changing a circle on the display from red to blue as you fully clean an area.

Higher-end cordless vacuums may also have companion apps that show things like battery level, filter status and cleaning logs. It’s an added level of convenience, but by no means necessary. Unlike robot vacuums, which rely on their apps to set cleaning schedules, manually control the machines and more, cordless vacuums that you operate yourself really don’t need Wi-Fi or an app connectivity.

Cordless stick vacuums range in price from $150 all the way up to over $1,000. The best ones for most people lie in the middle, in the $400 to $700 range. You’ll notice most of our picks land in the higher end of that range, but for good reason: More expensive machines tend to have more sucking power, which means less time wasted going over the same spots over and over. But does that mean everyone needs the most premium cordless vacuum? Definitely not. We’ve come up with top picks at various price points that should work well for people with different budgets, lifestyles, home sizes and more.

How we test cordless vacuums

Engadget doesn’t have a dedicated lab in which we can test cordless vacuums, but I used each model in my home for weeks. I ran them over hardwood and tile flooring, as well as low-pile carpet. And my first runthrough consisted of cleaning all three floors of my home on a single battery charge. I performed the same cleaning job as many times as possible, but also intermittently cleaned a single floor as needed, or sucked up isolated messes like crumbs, cat litter spills and tufts of pet fur. Over the course of many cleanings with each model, I made note of how loud the machine was, how easy it was to maneuver around my home, how easily it sucked up pieces of large debris (or if it pushed it around my floor instead) and if they got warm or hot.

Suction power: 240AW | Bin volume: 0.2 gallon | Battery life: 60 min | Weight: 6.83 pounds

The Dyson V15 Detect is just as impressive now as it was when it first came out in 2021 . It’s our top pick because it provides the best mix of features and its suction power is remarkable.

One thing that was immediately apparent during my testing was that Dyson machines just have superior suction, period. That’s not to say a cordless vacuum from another brand can’t get the job done, because they absolutely can. But even in auto mode, all of Dyson’s vacuums did a better job trapping even the smallest dust and debris.

The “Detect” in this model’s name refers in part to the laser “blade” on the Fluffy Optic cleaner head. It does a great job of illuminating floors to clearly show you where the mess really is, and that goes for large debris like food and granules of cat litter, as well as much smaller stuff like layers of dust on an untouched part of the floor. But it’s worth noting that this light is only present on the Fluffy Optic head, which is designed to work best on hard floors. The V15 Detect comes with a few other head attachments, including a Digital Motorbar head that automatically detangles hair from the brush bar as you clean. That’ll be better for all floor types, including carpet, but you won’t get the same illumination effect.

The V15 Detect also has a piezo acoustic sensor that recognizes the size and frequency of the particles it’s sucking up. It’ll show that information on the machine’s LCD display in a neat little bar graph. Is this information useful? Not particularly, but it’s satisfying to clock this while cleaning and see the bars increase and decrease as you move throughout your home. However, this is more of a neat perk than a killer feature; it’s not going to change the way you vacuum your home. But the piezo sensor is also involved in automatically adjusting the V15 Detect’s cyclone engine to better clean particularly dirty floors.

That automatic adjustment is separate from the machine’s three cleaning modes: Auto, Eco and Boost. I spent most of my time in Auto mode, every so often switching to Boost in rooms where I knew I needed a more thorough cleaning. Eco mode is convenient to have if you need to conserve battery life.

But the V15 Detect, like all of the other Dyson machines I tested, really stands out for its suction power. I rarely, if ever, had to go over the same spot twice because the V15 Detect captured all of the dust and debris the first time. In my one-cat household, it’s easy to see tufts of fur on our hardwood floors and tile, but they’re basically invisible on our upstairs carpet. But it didn’t matter if I could see fur or not — the V15 Detect collected all of it. After every cleaning the bin was full and I was consistently shocked by how much cat hair was hiding in my carpet.

As with most cordless vacuums, you get a number of attachments with the V15 Detect. In addition to the Fluffy Optic and Digital Motorbar cleaner heads, hair screw, combination and crevice tools are included in the box. I particularly like the hair screw tool, which is great for vacuuming chairs and couches, and the crevice tool is handy for cleaning car interiors and other tight spaces. The docking station must be mounted to a wall, but it’s separate from the vacuum’s charger, which means you can power up and use the V15 Detect before picking a permanent place for it in your home.

My biggest gripe with the V15 Detect is that it doesn’t have a single-button start like the newer and more advanced Dyson Gen 5 Detect and Dyson V15 Detect Submarine do. You must press and hold down the trigger to vacuum, which requires constant effort. However, that’s a small price to pay considering the complete package you’re getting. At $750, the V15 Detect is a machine on the higher-end of the price spectrum that’s well worth the money if you want an easy to use cordless vacuum that will provide a truly thorough clean every time.

  • Fantastic suction power
  • Includes Fluffy Optic cleaner head with illuminating blade light
  • Relatively lightweight
  • Good battery life
  • No single-button start
  • Headlight laser only on the Fluffy Optic cleaner head

Suction power: 500W | Bin volume: 0.12 gallon | Battery life: 40 min | Weight: 6.8 pounds

I reached for the Tineco Pure One S15 almost as much as I did for the Dyson V15 Detect. It’s slightly less powerful than our top pick, but it’s otherwise very similar and it even has Wi-Fi and app connectivity as well. It handled all kinds of dry messes well and didn’t choke at the first sign of pet hair. It includes Tineco’s iLoop smart sensor, which detects the amount of dirt and auto-adjusts suction power. I also appreciate that its LED display has a big ring that changes from red to blue as it sucks up all the debris in its way, and it has a battery indicator on it as well.

The Pure One S15 Pet has an unfussy design and comes with attachments that are easy to pop on and off. The main cleaning head has a headlight which does a great job of showing you exactly where the worst pockets of dirt are. And, despite its bin being a bit smaller than that on the Dyson, I never had to pause mid-cleaning to empty it.

Unlike the Dyson, though, the Pure One S15 Pet has Wi-Fi connectivity and a companion app, which is about as “smart” as any cordless vacuum gets. It’s a totally unnecessary feature, but Wi-Fi setup is pretty painless, and Tineco’s mobile app will show you things like battery level, filter status and a cleaning log. Filter status is arguably the most useful of them all, since it takes the guesswork out of figuring out when to replace it.

The $500 Tineco machine beat out the $450 Shark Detect Pro for our runner-up spot by only a hair. The Shark is a solid vacuum with a self-emptying base,in the same price range. But the Pure One S15 Pet has stronger suction and did a better job cleaning up big messes. It’s also worth noting that, while Tineco’s vacuum typically costs $500, you can often find it on sale for as low as $350 — a killer bargain.

  • Great suction power
  • Single-button start
  • iLoop smart sensor auto-adjusts suction power as you clean
  • Great value for the money
  • Wi-Fi and app connectivity
  • Shorter battery life

Suction power: 450W | Bin volume: 0.15 gallon | Battery life: 40 min | Weight: 5.7 pounds

Tineco makes a number of cordless vacuums at various price points, but the Pure One S11 provides a lot for $300 and it’s often on sale for less. While it was the clunkiest of the Tineco vacuums I tested, it’s still relatively lightweight. It also includes Tineco’s iLoop smart sensor, although it doesn’t have the single-button start that the Pure One S15 Pet does.

The Pure One S11 did a decent job of collecting all kinds of messes, including pet hair, but I did have to go over some spots more than once for the most thorough clean. In addition to the main cleaner head that has built-in headlights, it comes with three extra attachments that make it easy to get into small spaces.

Tineco estimates a run time of 40 minutes for this machine, and I found that to be accurate. That was just about enough time for me to clean all three floors of my house, but if you have a particularly large home, you may need to recharge at some point. Its bin size is also on the smaller side like the Pure One S15 Pet, so there’s a chance you may have to pause to empty it if you’re cleaning up big messes or you have a lot of ground to cover.

But for $300, the Pure One S11 offers a lot of value. It’s arguably best for those who live in apartments or small- to medium-sized homes, or those who just want a no-frills, easy-to-use cordless vacuum without dropping an excessive amount of money. It holds its own against pet hair as well, but that’s coming from a person who only has one cat; you’d be better off getting a stronger vacuum with a larger bin if you have a small menagerie in your home.

  • Good suction for the price

Suction power: 240AW | Bin volume: 0.2 gallon | Battery life: 60 min | Weight: 8.3 pounds

Overkill for most, the Dyson V15 Detect Submarine will be exactly what some are looking for: a vacuum with all of Dyson’s signatures along with the ability to wash hard floors. It’s similar to the V15 Detect in power and design, but it comes with Dyson’s “submarine” wet cleaner head that has two water reservoirs (one for clean water and one for dirty) and a brush bar that does all the scrubbing. To wash tile and other hard floors, you fill the clean reservoir with water, reattach it to the head and turn on the machine as if you were just vacuuming. The built-in motor ever so slightly propels the machine in this mop mode, and as you move it around, it sucks up spills and other wet messes into the dirty reservoir while also washing the floor.

In my testing, the V15 Detect Submarine did a great job sucking up spills and a pretty good job cleaning my hardwood and tile, even in spots where there were more persistent stains (although it did take a few passes to fully suss them out). I typically use a corded steamer to wash the hard floors in my home, and while the V15 Detect Submarine doesn’t use heat and can’t quite match up to a device that does, it’s impressive nonetheless for its abilities and convenience.

After my first go-around with the machine, I was excited to use it again primarily because the process is so easy. Instead of getting my steamer out, plugging it in and fighting with its cord as I moved around, I popped the Submarine cleaning head on the Dyson, filled the tank and off I went. It’s worth mentioning, though, that I did have to fill the clean-water reservoir twice to wash the main floor of my home (roughly 800 square feet), which added a few minutes to the process. After each session, you’ll want to empty both reservoirs, clean the brush head and let the whole attachment dry. After that, I found it was ready to go again the next day.

The V15 Detect is a solid vacuum and an above-average mop, but more than anything, it’s convenient. It’s one relatively thin and lightweight device that can clean all of the floors in your home without a ton of extra effort needed on your part and with few, if any, frustrations. But as with anything, convenience comes at a cost: you’ll pay $950 for this model, and no, you can’t just buy the Submarine head separately and use it with a standard Dyson stick vac. If you only have a little bit of tile or hard flooring in your home, it’ll be difficult to justify the cost of the Submarine — but if convenience is of utmost importance and you want one cordless vac-and-mop to rule them all, Dyson’s machine is a great option.

  • Includes Submarine cleaning head for mopping and cleaning up wet messes
  • Water tank is a little small

Suction power: 230W | Bin volume: 0.8 gallon | Battery life: 60 min | Weight: 5.29 pounds

Tineco’s high-end Pure One Station FurFree deserves a nod here because it was one of the cordless vacuums I tested that I wanted to use the most, and it’s arguably the most convenient option on the list. Not only does it have strong suction power and the company’s iLoop smart sensor, but it docks and charges in a self-emptying base that cleans all parts of the machine — brush, tube and dustbin — after each use.

The freestanding base is a little intimidating when you unbox it, purely because it has more parts than that of any vacuum cleaner I tested (including the Shark Detect Pro). The whole thing, vacuum included, remains relatively svelte; it was easy for me to tuck it into a corner of a room near an outlet. The vacuum itself has a single-button start and, while not quite as strong as the Dyson V15 Detect, did a good job cleaning up all kinds of messes, pet hair included. Run time comes in at 60 minutes, more than enough to clean my whole home, and it supports Auto and Max modes along with the auto-adjusting feature provided by the iLoop sensor.

The Pure One Station FurFree also supports Wi-Fi connectivity, with its mobile app showing you battery life, dustbin capacity and self-cleaning settings. But since the small screen on the docking station gives you most of that data as well, the app is just a bonus.

The biggest selling point of this cordless vacuum is its self-emptying base, which has its own large dustbin that can hold up to 60 days worth of dirt. It’s a plastic canister that takes no garbage bags, so there’s nothing to add to the cost of ownership — simply remove the base’s dustbin to empty into your own garbage can every couple of weeks and then snap it back into place. You can customize self-cleaning settings to do a quick clean (30 seconds long) or a deep clean (60 seconds), or keep it to the auto mode (45 seconds). The machine will clean itself whenever you return the vacuum to its dock, unless you manually disable this feature. I appreciate that the base automatically empties the vacuum’s dustbin during self-cleaning, but it’s also great that it uses gusts of air to clean the vacuum’s brush and tube. Lots of little particles and hair can get stuck in those parts, and with other vacuums, you’re left to clear those out yourself.

The “FurFree” in the Pure One Station’s name refers to the extra attachments that come with the machine that basically turn it into a pet-grooming tool. This will likely be more useful for dog owners, or maybe folks with more amenable cats than mine (she hates all vacuums — cordless, robot, you name it). If you’re not a pet owner, there’s a standard Tineco Pure One Station that’s otherwise an identical machine with the same self-cleaning base, but only comes with regular vacuum attachments, and costs $200 less. FurFree or regular, though, the Pure One Station is a great cordless vacuum that’s made better by its clever and convenient self-emptying base.

  • Includes self-emptying base
  • Base empties bin and also cleans the vacuum’s brush and tube simultaneously
  • FurFree model is expensive

Other cordless stick vacuums we tested

Dyson gen 5 detect.

The Dyson Gen 5 Detect has a single-button start and stronger suction power than our top pick, but it’s otherwise quite similar. However, since the Gen 5 Detect is more expensive at $950 (although it does receive discounts at Dyson online), the V15 Detect still provides greater value for your money. The Gen 5 Detect is arguably best for those who want the latest Dyson, or care about getting a more future-proof machine, since it came out just last year.

Shark Detect Pro

The Shark Detect Pro provides a lot of value for the money, but it was ultimately beat by the Tineco Pure One S15 Pet for our runner-up slot thanks to the Tineco’s stronger suction power. The kicker for the Detect Pro is that it includes a self-emptying base in its $450 price, which is super handy. It’ll automatically dump the contents of the vacuum into the larger bin in the base after every cleaning, and you only need to empty the base’s container every month or so. The Detect Pro did a good job cleaning up messes across different types of flooring, and it’ll auto-adjust suction power depending on the amount of debris and whether you’re cleaning hard or carpeted floors. However, it’s not as smooth to use as any of our top picks and its main cleaner head is a bit tall, making it difficult to use to clean under low furniture.

How long does the battery last on a cordless vacuum?

Most cordless vacuums will run for at least 30-40 minutes on a single charge, but you can find cordless vacuums with battery lives of up to 60 or 70 minutes. Manufacturers will outline an estimated battery life for each model, and they’re usually based on using the vacuum’s standard power mode for the entire runtime; if you switch between modes or prefer to use a higher-powered program for improved suction, you’ll drain the battery faster.

Can cordless vacuums be as powerful as corded ones?

Cordless vacuums do sacrifice a bit in overall power when compared to corded models, but that doesn’t mean they can’t handle everyday messes just as well. If suction power is your biggest concern, we recommend springing for a high-powered, high-end cordless vacuum since, typically, the more expensive a cordless vacuum is, the stronger the suction. Also, cordless vacuums have the edge over corded models when it comes to weight and convenience: cordless vacuums are much lighter than their corded counterparts, and you’ll never have to worry about placement or picking a fight with a cord while cleaning your living room.

Are cordless vacuums effective on pet hair?

Yes, cordless vacuums can handle pet hair well, but we recommend getting a model with strong suction power to get the best results. It’s also wise to get one with a larger bin, since pet hair can quickly fill up smaller bins, which may force you to stop cleaning to empty the vacuum before finishing.

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The 14 Best Acne Light Therapy Devices to Zap Stubborn Breakouts

Add one of these expert picks to your skin routine

best acne light therapy devices

Every product on this page was chosen by a Harper's BAZAAR editor. We may earn commission on some of the items you choose to buy.

Think of acne devices as the supporting cast in your treatment plan next to topicals. While the best acne light therapy devices are powerful enough to clear up breakouts, board-certified dermatologist Mamina Turegano tells Bazaar that in the derm world, LED devices are considered to be lower on the treatment scale, but can still be effective when paired with an expert-approved acne skincare routine. " Topical and prescription treatments are first recommended. A good foundational skincare regimen is important, followed by evidence-based tried and true medications, whether over-the-counter or prescription." Turegano notes that light tools can also serve as an alternative treatment for those who cannot tolerate or don’t respond well to conventional therapy like prescription topicals.

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Espada 2 Plus

Best for Recurring Acne

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TheraFace Pro

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Riki Miracle

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Riki miracle.

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Acne Light Therapy Patch

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Claro Acne Treatment Light Therapy System

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So how exactly does a light work clear skin? "LED blue light, targets the acne-causing bacteria cutibacterium acnes. The bacterium C. acnes produces porphyrins. These proteins absorb light energy in the spectrum of blue light. This leads to a chemical reaction that essentially destroys the bacteria," she explains. It's always best to chat with a dermatologist before starting a new skincare treatment. The efficacy of an acne light therapy device depends heavily on the type of acne you're looking to treat. "With mild acne, LED skincare can even be superior to many topical regimens, especially for those with sensitive skin who get easily irritated by standard medicines," Ellen Marmur , board-certified dermatologist, and founder of MMSkincare says. "For severe acne, LED tools and devices are additive to the skincare routine rather than a replacement," she adds. Once you're ready to try one of the best acne light therapy devices, look to one of the following expert-recommended options below.

This handy LED light nicely fits in your bag and combines red and blue lights for a zit-zapping effect that works in three minutes. And did we mention it's completely painless?

One satisfied customer writes: "Awesome product!! I love how slim and portable this is. Works great for the deep acne. This kills the bacteria growing inside and allows your body to heal it without it having to erupt! Easy to use and doesn't dry my skin out." — Gloria

The small but mighty SolaWave Bye Acne Spot packs enough power to treat a pimple in three minutes flat— we've tested it first-hand . It combines red and blue light treatments to clear pores while calming skin at the same time.

One satisfied customer writes: "I love how compact this product is it makes it really easy for me to travel with it and use it on the go. The packaging is great quality is amazing and I’ve noticed that it helps minimize blemishes after just a few uses." — Jess

For breakouts that cover more than just a spot but not your entire face, try this Acne Light Shot from Face Gym. It comes with 60 medical-grade adhesive strips that'll stay put while treating and goggles to protect your eyes from any light sensitivity.

One satisfied customer writes: "This has been the only thing that has helped clear up my cheek acne. I also use retinol and low percentage benzoyl peroxide, but adding this has made a huge difference. " — Christiebelle

Foreo's handy Espada 2 Plus tool is perfect made for more targeted treatments. The tapered tip directs the light right to a specific blemish or spot where recurring pimples pop up to kill bacteria beneath the skin's surface. The device also features ultrasonic vibration to stimulate cell turnover and get rid of scarring.

One satisfied customer writes: "The precise light guide itself is quite small (you get exactly what you see in the pictures), hence making it a perfect device to treat small, curved, and hard-to-reach areas like around the nose, or right below the eyebrow." — Emily E.

Turegano recommends this eight-in-one device—one that Bazaar editors also love —which has detachable treatment heads to clear away pimples. It also helps bring down inflammation, firm less than taut skin with its built-in microcurrent , and provides a nice facial massage at the same time.

This pick from Dr. Dennis Gross may look intense, but it's probably the closest to a professional facial that you'll get at home. The mask features 62 blue LED lights to treat and prevent acne-causing bacteria and 100 red LED lights to help minimize acne scarring and boost collagen production. You can treat skin with each light shade individually or at the same time.

One satisfied customer writes: "Not only is this device user-friendly with its hands-free design and adjustable strap, but the impact on my skin has been remarkable. The combination of red and blue lights proved to be a dynamic duo in combating both wrinkles and acne. Surprisingly, within days, I noticed a visible reduction in new acne development, and over the course of two weeks, my mild hyperpigmentation from previous breakouts is beginning to fade." —Victoria B.

Consider this the LED version of your favorite hydrocolloid pimple patch . The device comes with sticky patches that help it adhere to skin. Each treatment is six minutes long and emits blue and red light to kill bacteria and bring down inflammation. Plus, it has a rechargeable battery and a storage case to keep it clean.

One satisfied customer writes: "The results are noticeable immediately after one use. I have hormonal acne and this product has been my saver! It’s very easy to use, and you don’t have to stop what you’re doing because it’s hands-free. Love it!" — Malena

Designed with a combo of 36 blue and red LEDs, Lightstim for Acne is best used to treat mild to moderate breakouts. The tool is also ideal for keeping your skin in the clear to prevent new acne from forming.

One satisfied customer writes: "Since the start of the pandemic, I've been getting really bad hormonal breakouts along my chin. I'm in my late 20's and never suffered acne like this before. It's been deep cystic pimples that would leave behind bad scarring. I've tried a few different products, and the LightStim has so far been the best." — LAinLALA

Lab's single blemish LED works similarly to other options on this list, except this one is under $20 and works for 30 treatments before needing to be replaced.

One satisfied customer writes: "It's pretty great, and a single box lasts a long time. Used this on Friday, and woke up Saturday with clearer skin. I was skeptical, but the results were real!" —Alex

While several other tools on this list integrate both blue and red LEDs into one head, this option from Spa Sciences has separate treatment heads, each with a larger surface area than most devices for a more concentrated treatment of either acne or the post-acne scarring that can come with it.

One satisfied customer writes: "I have been using this for a little over 2 months and my favorite part is the reduction of inflammation from the breakouts I have had. It definitely seems to reduce the redness and speed healing time!" —Sandra

MM Skincare MMSphere 2Go

MMSphere 2Go

Marmur created this multi-purpose device to treat a range of skin concerns, including acne. The device features a full range of light wavelengths with different colors (blue, red, green, purple, and amber) to improve acne, prevent breakouts, and reduce redness and acne scars. It's also portable so you can treat skin even while traveling.

One satisfied customer writes: "I had a friend today tell me my skin looked the best he'd seen it in ten years. After 6 weeks of using the Sphere, I'm never going to stop. Combined with the MMRepose serum I've found my scarring reduced, my skin consistently clearer and feeling more taut and bouncy. " — Duncan M.

Revive Light Therapy Glō Portable LED Light Therapy Device

Glō Portable LED Light Therapy Device

In addition to bacteria-killing blue light, Revive's Glō device also featured infrared shades of deep red, red, and amber lights for more thorough inflammation reduction. Plus, the size of the tools makes for easy treatments while traveling.

One satisfied customer writes: "This is my new favorite skincare tool! I have used it every day for the past few weeks and I have noticed less acne and a reduction in my fine lines. I love that I can easily use this while I am working at my computer, and take this with me when I travel." —Deborah P.

PMD Clean Acne

Clean Acne

PMD's Clean Acne device not only checks the cute box, but it fills the space of two tools in one. The silicone brush side uses sonic vibration to clear away grime and bacteria, and when flipped, the blue LED does the deep pore purging.

One satisfied customer writes: "I use it every night to clean my face and then use the blue light on my problem areas. Finally, I’m getting my healthy, clear, glowing skin back!" —Alina

Clear LED Mask

Clear LED Mask

This device from Omnilux features a flexible silicone design that treats the entire face in one fell swoop. It contains 132 professional-grade LED lights to clear up acne and soften the appearance of post-breakout scars. The mask even has built-in eye shields to protect your eyes as you treat your skin.

One satisfied customer writes: "Omnilux is clearly doing its magic. I’m so excited to finally be acne-free for the first time in my entire life. This is coming from someone who tried everything on the planet since middle school." —Zack

.css-1jvcbpx{font-family:NewParisTextBook,NewParisTextBook-roboto,NewParisTextBook-local,Georgia,Times,Serif;font-weight:normal;letter-spacing:-0.015rem;margin-bottom:0.625rem;margin-top:0.625rem;}@media(max-width: 48rem){.css-1jvcbpx{font-size:2.25rem;line-height:1.1;}}@media(min-width: 48rem){.css-1jvcbpx{font-size:2rem;line-height:1.1;}}@media(min-width: 64rem){.css-1jvcbpx{font-size:2.625rem;line-height:1.1;}}.css-1jvcbpx b,.css-1jvcbpx strong{font-family:inherit;font-weight:bold;}.css-1jvcbpx em,.css-1jvcbpx i{font-style:italic;font-family:inherit;} Meet the Experts

a thick linebreak in a guide to the best oversized tote bags

  • Mamina Turegano, M.D., F.A.A.D, is a triple board-certified dermatologist based in Metairie, Louisiana.
  • Ellen Marmur, M.D., is a board-certified dermatologist and founder of Marmur Medical and MMSkincare, based in New York City.

When in my routine should I use one of the best acne light devices?

best acne light therapy devices

Marmur notes that best time to use an acne light device is after cleansing your face as "debris and dry skin cells can block the light energy." Acne light therapy devices work on most types of acne, but won't do much for blackheads , whiteheads, or deep cystic pimples, Turegano cautions—so stick to topical treatments and oral medication for those. And before your session, the derm says to avoid using products that exfoliate or cause increased skin cell turnover, like exfoliating acids or retinoids , as they may make your skin more sensitive.

Why trust Harper’s Bazaar ?

For more than 150 years, Harper’s Bazaar has been the preeminent fashion and beauty resource for women at every age. We cover what’s new and what’s next in beauty by working with the world’s leading authorities in dermatology, plastic surgery, cosmetics, skincare, hair care, and fragrance. Every story we publish has been thoroughly researched and vetted by our team of beauty editors and industry experts.

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  • Deals and sales

The 80+ Best Deals From Amazon's Big Spring Sale (And Across the Internet) This Week

collage of earbuds, sneaker and toaster

It's the last day of the  Big Spring Sale , with Amazon's newest manufactured sales event promising continued savings on cleaning gear, outdoor essentials, electronics, clothing, and home goods until midnight Pacific time.

We've assessed the deals on offer. Most didn’t make the grade, and many have ended already, so we once again dug in to find those that are truly worth the spend.

Below, we’ve rounded up the best remaining deals from Amazon and other retailers offering competing sales and discounts to find you only the best  spring deals  on our expert-approved picks.

The best deals on:

Lawn and garden, outdoor, camping, and hiking, luggage and travel, kitchen and appliance, tech, laptops, and tv, spring clothing and shoes, home, furniture, and personal care.

A fire burning in the Solo Stove Bonfire 2.0 fire pit, which is sitting on gravel, with a desert in the background.

Solo Stove Bonfire 2.0 Fire Pit — Our Pick Target deal price: $200 in cart ; street price: $250

What we like: Our lightweight smokeless fire pit pick for moving in and out of storage. Also recommended in multiple Wirecutter gift guides. Simple and easy to load. Half the weight of our other picks. Removable ash pans and grates make it easier to clean. Radiates less heat than other heavier models.

Other things to know: Plain aesthetic won't be appealing to everyone. No “smokeless” fire pit is truly smokeless, but smoke is reduced by 70% to 80%. Price reflects in cart. Target RedCard holders save the typical additional 5%.

Read our ode to the Solo Stove Bonfire .

Eley 5/8-inch Polyurethane Garden Hose (50 Feet) — Upgrade Pick Eley deal price: $103 ; street price: $122

What we like: Our upgrade garden hose pick. Much lighter, easier to loop, more durable, and resists folding or kinking compared to other options. Large brass fittings at the ends are easy to tighten by hand or wrench. Absolutely hosetastic—the best we’ve ever tested.

Other things to know: Pricey—even when on sale—but if you want a hose that's better in every way and are willing to pay for it, this is the one to get. 10-year warranty. Deal price is for the 50-foot length.

Read our review of the best garden hose and hose reel .

Fiskars 28" Power-Lever Bypass Lopper — Also Great Target deal price: $20 with store pickup or Target RedCard ; street price: $23

What we like: A larger variant of the lopper pick in our guide to lawn and garden gear to get your yard cleaned up. A lopper cleanly cuts branches too big for pruners, and because it requires just one simple snip, it works far faster than a saw. It’s great for dealing with low-hanging limbs.

Other things to know: Might not be necessary if you already have a pruners and saw. This 28-inch version is good for bigger and harder-to-reach branches. With store pickup. Target RedCard holders get free shipping and save an extra 5%.

Read our review of the best lawn and garden gear to get your yard cleaned up .

Govee Outdoor String Lights (96 Feet) — Another Good Option Amazon deal price: $65 with clipped on-page coupon ; street price: $90

What we like: Another good string light in our guide to the best smart outdoor lighting. Affordably priced and available in a 48-foot set and a 96-foot set. They provide far more special-effects options than our string light pick.

Other things to know: Very overwhelming app. The strands have fewer bulbs per foot than our string-lights pick, and they don’t feel as sturdy. Frosted bulbs. Deal is for the 96-foot length; be sure to clip the on-page coupon.

Read our review of the best smart outdoor lighting .

Ego LM2135SP Power+ 21″ Select Cut Lawn Mower with extra battery — Runner-Up Lowe’s deal price: $600 with store pickup ; street price: $950

What we like: Our runner-up pick for the best lawn mower, bundled with an extra Ego battery. Battery-powered self-propelled mower can run for about an hour on a single battery charge. No need for gas, oil checks, or fuel storage, and no noxious emissions. Considerably quieter than gas mowers.

Other things to know: First deal we’ve seen since Black Friday. Minimal maintenance compared with gas models. A little less powerful and shorter run time than our top pick, but additional battery easily makes up for primary battery’s capacity. Extra Ego BA2800T battery has 56-volt, 5 Ah, 280 Wh capacity, usually between $200 and $300 when purchased separately. Store pickup required.

Read our review of the best lawn mower .

A spray bottle of Sawyer premium insect repellent.

Sawyer Products Permethrin Fabric Treatment (24 Ounces) — Also Great Amazon deal price: $14 ; street price: $18

What we like: Our bug repellent pick for clothing and gear (but not your skin). As effective as similar formulas at repelling ticks and mosquitoes, and its trigger spray is easier to control and apply evenly. Each application provides about six washings (or six weeks, whichever comes first) of protection.

Other things to know: The 24-ounce bottle should be enough to treat four outfits (shirt, pants, socks). The set of two 24-ounce bottles offers a slightly better price per ounce if you need to stock up.

Read our review of the best bug repellents .

Yeti Tundra Haul Hard Cooler — Our Pick Moosejaw deal price: $340 with single-use discount and free Dick's ScoreCard account ; street price: $425

What we like: Our pick for the best cooler with wheels. Easy to haul across long distances. Durable wheels. Thick-gauge welded aluminum handle won't strain or bend. Feels similar to wheeling around high-end luggage.

Other things to know: No internal divider. Contents may shuffle a bit when tipped at an angle. Available in six colors. You have to sign in with your free Dick's ScoreCard account to get the discount on a single full-price item from Moosejaw. Shipping is free.

Read our review of the best coolers .

Supergoop Play Everyday Lotion SPF 50 (18 Ounces) — Our Pick Supergoop deal price: $54 with code WC24 ; street price: $68

What we like: One of our chemical sunscreen picks that's available in bulk. Good for face and body. Rubs in easily and is nearly transparent once applied. Light, pleasant scent. SPF 50 rating offers a high level of protection. User-friendly pump bottle.

Other things to know: None of Wirecutter’s testers experienced a white cast, but some reported that it left them feeling greasy. The 18-ounce jug is the most affordable size on a per-ounce basis. Use code WC24 .

Read our ode to Supergoop sunscreen .

NiteRider Swift 300 Bike Light — Budget Pick REI deal price: $24 with single-use code MEMBER24, REI membership required ; street price: $30

What we like: Our headlight budget pick for the best commuter bike light is small and sturdy, with a swiveling strap and decent side visibility. Can emit 300 lumens for up to two hours and flash for 12. Adjusts to fit most sizes and shapes of handlebars.

Other things to know: Not suited to longer or darker routes. Lifetime warranty against manufacturing defects, one-year warranty on its rechargeable battery cells. Already affordable but rarely sees discounts. On sale for REI members with single-use promo code MEMBER24.

Read our review of the best commuter bike lights .

Blundstone Original 500 Series Boot — Staff Favorite REI deal price: $168 with single-use code MEMBER24, REI membership required ; street price: $210

What we like: A casual leather boot loved by Wirecutter staff and many others around the world. Pull-on, ankle-high, with an elastic panel. Comfortable, durable rubber sole. Uppers made of extra-thick leather. Ergonomically engineered to reduce fatigue. Can last for years with daily wear.

Other things to know: Exceptionally light for a pair of boots, about 14 ounces per pair. Unisex. Best price and size availability is for the stout brown color. On sale for REI members with single-use promo code MEMBER24.

Read our ode to the Blundstone boots .

Panel Sound USAPA Approved Fiberglass Pickleball Paddles — Staff Favorite Amazon deal price: $20 in select colors ; street price: $35

What we like: A staff-favorite pickleball paddle set. Lightweight and comfortable to hold. Grippy handles and rubbery faces absorb some of the ball’s velocity. Good for beginners.

Other things to know: Set includes two paddles, four indoor balls, a carrying case, and two cooling towels. Deal is for the red paddle set.

Read our review of the best pickleball equipment for playing at home .

Vasque Breeze Waterproof Hiking Boot (Men’s Sizes) — Also Great REI deal price: $96 with single-use code MEMBER24, REI membership required ; street price: $140

What we like: Our more flexible hiking boot pick. Offers all the comfort of a trail-running shoe with the ankle support of a hiking boot. Lighter weight (just over 2 pounds per pair) is good for longer hikes. Substantial enough to support you while carrying a heavy pack.

Other things to know: Runs narrow—we recommend ordering at least a half-size up from your street-shoe size. Takes longer to break (about 10 miles) in than our top pick. On sale in nutria color for REI members with single-use promo code  MEMBER24 .

Read our review of the best hiking boots .

Snow Peak Ti-Single 600 Cup With Hotlips — Staff Favorite REI deal price: $34 with single-use code MEMBER24, REI membership required ; street price: $43

What we like: A larger version of an ultralight titanium mug we like for campfire cooking and making coffee on the go. Durable, smooth titanium design. Roomy flip-out handle. Attach to a carabiner for external storage. Food-grade silicone HotLips help mitigate potential scorching.

Other things to know: Cup may be too hot to drink from on its own at first pour. Can also be used as a single-serving pot for boiling water. Hotlips only fit on Snow Peaks mugs. Use single-use coupon code MEMBER24 . REI membership required.

Read our review of our favorite campfire cooking tips tricks and gear .

Read why the Solo Stove Bonfire is our favorite smokeless fire pit .

Someone walking with a Cotopaxi Allpa 35L Carry-On Travel Backpack on.

Cotopaxi Allpa 35L Carry-On Travel Backpack — Our Pick REI deal price: $160 in select colors with single-use code MEMBER24, REI membership required ; street price: $200 Moosejaw deal price: $160 in select colors with single-use discount and free Dick's ScoreCard account ; street price: $200

What we like: Our pick for the best small carry-on travel backpack for most situations. Easy-to-pack clamshell design and highly adjustable straps. Has handles on all four sides. Protected by a full lifetime warranty and has the build quality to back that up. Made with 1,680-denier ballistic nylon and includes a rain fly.

Other things to know: Minimal amount of administrative organization. We’d love to see a permanent all-nylon alternative to the TPU-lined front panel, which can flake over time. On sale in the river color for REI members with single-use promo code MEMBER24 or for the public at Moosejaw with a single-use promo and free Dick's ScoreCard account.

Read our review of the best carry-on travel backpacks .

Monos Hybrid Trunk — Also Great Monos deal price: $410 with code ANOTHERYEAR ; street price: $455

What we like: Our pick for the best trunk for checking. Made of a robust polycarbonate with aluminum corner guards and an aluminum frame around the center seam that reinforces the body of the luggage. Has two TSA locks, which close with a satisfying latch.

Other things to know: Due to the design, some travelers consider trunks to be easier to pack and to maneuver. Does away with the standard zipper closure. Doesn’t have a ton of built-in organization. Lifetime warranty. Use code ANOTHERYEAR . Sold out in silver, but available in champagne and black colors.

Read our review of the best suitcases for checking .

Apple AirTag Bluetooth Tracker (4-Pack) — Top Pick Target deal price: $80 ; street price: $100

What we like: Our pick for the best Bluetooth tracker for iPhone users. Impressively accurate in pinpointing an item’s precise location. Leverages Apple’s extensive Find My network. Can swap in a new battery as needed. Water-resistant.

Other things to know: Doesn’t have any way to ring your phone. Cannot attach to your stuff directly. Cannot be shared with another person. Additional 5% savings available for RedCard members.

Read our review of the best Bluetooth tracker .

Apple AirTag Bluetooth Tracker (Single) — Our Pick Amazon deal price: $24 ; street price: $29

What we like: Our pick for the best Bluetooth tracker for iOS users leverages Apple’s huge network of devices to find lost goods. Impressively accurate. Replaceable coin battery. If you lose an AirTag (or an item containing or attached to one), you can use the Find My app to see its location or ask Siri to trigger an audible alert.

Other things to know: Doesn’t have any way to ring your phone. No built-in method of attaching to anything. Solid deal for an individual tracker, though the sale for a set of four above offers a better price per tracker.

Bellroy Tokyo Tote Second Edition — Our Pick Amazon deal price: $89 in select colors ; street price: $129

What we like: Our pick for a durable, water-repellent polyester laptop tote bag. Tough yet polished. Transitions perfectly from the coffee shop to the conference room. Thoughtfully designed, with features like internal popout bottle pockets and leakproof zippers.

Other things to know: Fabric shell is abrasive and may make some sweaters and blouses pill. The weave also attracts pet hair like a magnet. Only available in ranger green. Lowest price we've seen so far.

Read our review of the best laptop tote bags .

The Breville Smart Oven

Breville Smart Oven Air Fryer Pro — Top Pick Amazon deal price: $320 ; street price: $400

What we like: Our pick for the best air fryer toaster oven, and our upgrade pick for the best toaster oven. Performs just as well as an air fryer as it does a toaster oven. Specific settings for dehydrating, proofing, and slow cooking. Crisps up food faster than traditional toaster ovens. Large two-rack capacity. Multiple fan speeds.

Other things to know: Even on sale, notably more expensive than our other picks. "Super convection" fan speed is incredibly loud. Didn't toast bread quite as evenly as other picks. Very heavy, and we'd prefer a pizza stone over the included nonstick pizza pan. Matches previous low.

Read our review of the best air fryer toaster oven .

Simple Modern Classic Tumbler (28-Ounce) — Top Pick Amazon deal price: $21 ; street price: $26

What we like: Our upcoming new pick for the best tumbler. Affordable and leak-resistant. Stainless steel and double-walled; keeps hot or cold drinks at an ideal temperature for hours. Only tumbler we tested that comes with both a straw lid and a flip-top lid.

Other things to know: Dishwasher-safe. Sale is on several colors, including black, dark blue, and almond birch.

Breville Smart Oven Pizzaiolo — Also Great Amazon deal price: $800 ; street price: $1,000 Williams Sonoma deal price: $800 ; street price: $1,000

What we like: Our pick for the best indoor countertop pizza oven. Preheats up to 750 °F in 15 minutes and can cook a pizza in just over 90 seconds. Comes with preset functions, precise temperature control, a timer, pizza peel, and pan.

Other things to know: A costly, specific-use appliance. Almost twice the price of our top outdoor pizza oven pick when not on sale. Doesn't get as hot as our outdoor oven picks. Deal matches previous low, though that $800 price is a recurring one.

Read our review of the best pizza oven .

Cuisinart Frozen Yogurt-Ice Cream & Sorbet Maker (ICE-21) — Top Pick Amazon deal price: $56 ; street price: $60

What we like: Our top ice cream maker pick. Made some of the smoothest and most delicious ice cream in our tests. More affordable than a compressor model. Churns ice cream faster than the competition.

Other things to know: Bowl insert must be pre-frozen overnight, which requires planning. Extra freezer bowls, required to make multiple batches in a row, sold separately.

Read our review of the best ice cream maker .

OXO Easy-Clean Compost Bin (1.75 Gallon) — Pick Variant Bloomingdale's deal price: $26 in cart ; street price: $32

What we like: A larger version of our simple, compact compost bin pick. A lightweight, no-nonsense plastic pail. Easy to clean. Hinged handle for carrying.

Other things to know: Attached lid isn't airtight. Worked well in our freezer (to go a couple weeks between compost drop-offs). Price reflects in cart.

Read our review of the best kitchen compost bins we love .

OXO Good Grips Glass 9" Pie Plate with Lid — Top Pick Bloomingdale's deal price: $12 in cart ; street price: $15

What we like: Our top pick for the best pie plate. Simple and sturdy. Holds more filling than the competition. Handy lid allows for easier storage and transport. Minimalist design.

Other things to know: Made of borosilicate glass. Two inches deep. A little too big for store-bought crust. Price reflects in cart; free shipping.

Read our review of the best pie plate .

Thermos Funtainer Food Jar (10 Ounces, 2-Pack) — Also Great Amazon deal price: $15 for a 2-pack ; street price: $22

What we like: A two-pack of our kids pick for best food thermos. Durable and easy for small hands to open. Never leaked in our tests, and the lid never cracked during drop tests.

Other things to know: 10-ounce container is best for kids (or for a small adult lunch). Doesn't retain heat for as long as our top pick (130 °F after 6 hours, vs 144 °F for our top pick). Best suited for keeping foods hot, not great for keeping foods cold. Deal price is for a 2-pack.

Read our review of the best food thermos .

YouCopia StoraLid Lid Organizer (Large) — Staff Favorite Amazon deal price: $16 ; street price: $20

What we like: A staff-favorite lid organizer we like for keeping things tidy in the cabinet. Keeps food-storage container lids organized and easily accessible with minimal human effort. Adjustable tabbed dividers for separating lid sizes.

Other things to know: Only as useful as your willingness to use it and organize. Needs large empty space to fit into a cabinet. Deal for the large size, recurring sale price.

Read our ode to the YouCopia StoraLid Lid Organizer .

Atlas Coffee Club 2-Bag Subscription — Gift Pick Atlas Coffee Club deal price: $14 with code WCMARCH50, new subscribers only ; street price: $28

What we like: A coffee subscription service we recommend in multiple gift guides. Atlas Coffee Club delivers satisfying coffee at a low price, with plenty of customization, and you always have something new to try. Sources beans from countries all over the world. Each bag of coffee includes a postcard with fun facts about the country’s coffee industry.

Other things to know: The subscription is highly customizable. New subscribers can use code WCMARCH50 for 50% off their first two-bag subscription. Matches the best previous pricing we've posted.

Read our review of the best gifts for coffee lovers .

OXO Brew 8-Cup Coffee Maker — Another Good Option Amazon deal price: $140 ; street price: $180

What we like: A speedy drip coffee maker we like that can make pour-over-quality single servings in addition to full pots. Small footprint and simple design. Brews coffee with well-rounded flavor. Can brew directly into the thermal carafe or a mug.

Other things to know: Lacks auto-brew function, clock, and timer.

Read our review of the best drip coffee maker .

Philips GoZero Sparkling Water Maker — Budget Pick Amazon deal price: $54 ; street price: $64

What we like: Our budget soda maker pick has a sleek and attractive no-frills look. Makes good-tasting seltzer with lively bubbles. Surprisingly good quality for the price.

Other things to know: Assembly can be a pain. CO 2 cylinder is sold separately for around $30. If you have empty cylinders, you can exchange them for a discount.

Read our review of the best soda maker .

Tovolo Dino Popsicle Molds (Set of 4) — Also Great Amazon deal price: $10 ; street price: $16

What we like: A small-ish dinosaur-shaped popsicle mold we love for kids (or dino-loving adults). Slightly smaller size and cute shape are perfect for fun frozen treats and experimenting with new flavors. Dishwasher-safe, so they're super easy to clean.

Other things to know: A little fussy to fill. Silicone mold is easy to peel off frozen pops—doesn't require pre-soaking or rinsing in warm water.

Read our review of the best popsicle molds .

Cuisinart CPT-122 2-Slice Compact Plastic Toaster — Top Pick Amazon deal price: $24 in select colors ; street price: $30

What we like: Our longtime top pick for the best two-slot toaster. Produced the most evenly browned bread in our testing. Wide range of settings and easy-to-use controls. Wide slots for thick-cut bagels.

Other things to know: Three-year warranty. Available in white. Plastic body and levers don’t look quite as nice as a metal finish. Heating-element wires of modern toasters are subject to corrosion over time.

Read our review of the best toaster .

Cuisinart CPT-142 4-Slice Compact Plastic Toaster — Top Pick Bloomingdale's deal price: $40 in cart ; street price: $50

What we like: Our longtime top pick for the best four-slot toaster. Consistently produces four slices of perfectly golden brown toast. Offers a wide range of settings and easy-to-use controls. Housing remains cool to the touch, even after multiple batches.

Other things to know: Heating element wires of modern toasters are subject to corrosion over time. Only available in white. Price reflects in cart.

One of our picks for the best cordless stick vacuum, the Ryobi 18V One+ HP Cordless Pet Stick Vac Kit with Dual-Roller Bar, standing on its own.

Ryobi 18V One+ HP Cordless Pet Stick Vac Kit with Dual-Roller Bar — Our Pick Home Depot deal price: $225 ; street price: $300

What we like: Our heftier cordless stick vacuum pick. Sturdy and versatile. Has several useful attachments and a large dustbin (twice the capacity of our Dyson pick). Did an excellent job of picking up all kinds of debris off different types of surfaces. Charges especially quickly. Has a generous three-year warranty and replaceable parts.

Other things to know: Heavy, and it may be uncomfortable to use. Ryobi products are sold exclusively through Home Depot and we've noticed stock issues with vacuum-only model. Ordering replacement parts is inconvenient. Only returnable in store.

Read our review of the best cordless stick vacuum .

Dyson V8 Origin+ Stick Vacuum (Refurbished) — Another Good Option eBay deal price: $168 ; street price: $300

What we like: Another good option when considering Dyson model stick vacuums. Less powerful than the V12 Detect Slim and the V15 Detect, but still a great value. Its bin capacity lies between that of the other two models, and it comes with useful attachments. Provides up to 40 minutes of power on full charge. Dyson is a reliable brand with a well-earned (if particularly devoted) following.

Other things to know: Equipped with docking station, Motorbar cleaner head, combination tool, and crevice tool. Certified refurbished and sold by the Dyson eBay storefront; includes a two-year Allstate warranty. Use code DYSONDIRECT at checkout for the deal price.

Roborock Q5+ Robot Vacuum — Our Pick Roborock deal price: $370 with code ROBOROCKS ; street price: $550

What we like: Our self-emptying robot vacuum pick. Better battery life, app navigation, mapping, and voice command response to Amazon Alexa and Google Assistant than other bots. Large bin. Excels at cleaning hard floors and low- to medium-pile carpets.

Other things to know: Comes with a charging station that empties debris into a disposable bag. Like nearly all bots we tested, it struggles with debris near thresholds and baseboards. A new low price, but not by much. Use coupon code ROBOROCKS .

Read our review of the best robot vacuums .

An Apple MacBook Air.

Apple MacBook Air (M2, 2022, 13″, 8 GB RAM, 256 GB SSD) — Top Pick Best Buy deal price: $900 ; street price: $1,000 B&H deal price: $900 ; street price: $1,000

What we like: Our top pick for the best MacBook. Compared with previous generations has a brighter display, better webcam, MagSafe magnetic port, and other design updates that make for an overall better experience. Great option for everyday work tasks, browsing the web, watching movies, and playing App Store games.

Other things to know: We expect to see better deals in the coming year now that the M3 processor is out, but still a worthwhile pickup if you need a laptop now or aren't worried about having the fastest processor. Not ideal for editing photos, videos, or processing large media files for extended periods. Specs include the newly dethroned M2 processor, 8 GB RAM, and 256 GB SSD.

Read our review of the best MacBooks .

Anker PowerConf C200 2K Webcam — Budget Pick Amazon deal price: $48 ; street price: $60

What we like: Our budget pick for the best webcam. Easy to set up. Provides 2K HD resolution. Offered the best image quality of any webcam we tested under $70. Has a switch to use the privacy screen. Software is basic but works well.

Other things to know: Sharpness, autofocus, and auto white balance aren’t as good as that of our top picks. Can’t swivel from side to side.

Read our review of the best webcams .

$100 Xbox Gift Card (Email Delivery) — Pick Accessory Newegg deal price: $90 ; street price: $100

What we like: Essentially free money toward your next purchase from the Microsoft, Windows, and Xbox online storefronts. Great for games and accessories associated with our Xbox game console picks. Can also be used towards the purchase of devices, apps, movies, and TV shows.

Other things to know: Use promo code SSDQ2545 at checkout for deal price. Email delivery. Non-returnable and non-refundable.

Read our review of the best game consoles .

$100 Apple Gift Card + $10 Target Gift Card (Email Delivery) — Pick Accessory Target deal price: $100 ; street price: $110

What we like: A gift card that can be used for purchasing Wirecutter-recommended Apple products, accessories, and apps. The $10 Target gift card is essentially free money for Target shoppers when you purchase the $100 Apple gift card.

Other things to know: Apple gift card can be used for purchases at Apple Store locations, as well as on the Apple website and App Store. Limit one Target gift card per customer. Cannot be returned or redeemed for cash or credit.

Read why we think the iPad is the best tablet .

Soundcore Space One Noise-Cancelling Headphones — Budget Pick Amazon deal price: $80 ; street price: $100

What we like: Our budget pick for the best over-ear noise-cancelling headphones offer impressive noise cancellation and good sound for less than $100. Comfortable and lightweight. 40-hour battery life with ANC on. Offers dual-device connectivity, hear-through mode, and quick-charge feature.

Other things to know: The three noise-reducing microphones can make voices sound compressed and somewhat distant to callers. Sound is customizable via the Soundcore app.

Read our review of the best noise-cancelling headphones .

Anker Soundcore Space A40 Earbuds — Top Pick Amazon deal price: $50 ; street price: $80

What we like: Our top pick for best wireless Bluetooth earbuds. Solid sound quality, great battery life, and active noise cancellation. Each earbud can be used independently. Dual-device connectivity. Should fit most ears comfortably.

Other things to know: Does not support voice-activated assistants like Alexa and Siri. IPX4 water-resistance. 18-month warranty. Limited touch controls. This deal lacks the $5 gift card we typically see bundled with these earbuds.

Read our review of the best wireless Bluetooth earbuds .

Acer Aspire 3 Spin 14 (A3SP14-31PT) — Top Pick Amazon deal price: $350 ; street price: $420

What we like: Our top pick for the best Windows laptop under $500. Fast enough to meet most people’s computing needs for a few years to come. 8.5-hour battery life. Accurate and reliable keyboard and trackpad. 360-degree hinge allows you to flip the display around for use as a tablet.

Other things to know: No fingerprint reader. Mediocre webcam. Shipped with Windows 11 S mode, but you can switch to Windows 11 Home for free. Specs include Intel Core i3-N305 processor, 8 GB of RAM, and 128 GB SSD.

Read our review of the best laptop under $500 .

Sony PlayStation 5 Slim Digital Edition + Marvel's Spider-Man 2 Bundle (No Disc Drive) — Also Great Target deal price: $400 ; street price: $450 Best Buy deal price: $400 ; street price: $450

What we like: The new "Slim" version of our top PlayStation game console pick without a disc drive, plus the new Spider-Man 2 2 game we loved in our recent review . Slightly narrower and shorter than the OG. 1 TB storage (up from 825 GB).

Other things to know: The Digital Edition doesn’t include a disc drive, so you can’t watch Blu-rays or take advantage of new or used games, or old PS4 discs. Still some backwards compatibility issues with PS4 games and exclusives. No Dolby Vision or or DTS:X support. Despite being called the "Slim," it's still quite large and bulky. Marvel's Spider-Man 2 digital code included.

Fujifilm Instax Mini Link 2 Instant Photo Printer — Top Pick Best Buy deal price: $90 ; street price: $100 Target deal price: $90 ; street price: $100

What we like: Our top pick for best instant photo printer. Ultra-portable and compact. Quickly delivers vibrant, credit-card-sized film-like prints. Easy to set up and use with an intuitive app.

Other things to know: Printer body feels fragile. Prints take four to six minutes to fully develop and may lack fine detail and clarity. Uses the same film packs as Fujifilm’s Instax cameras. Recurring deal price, but has yet to drop lower. Available in black, white, and pink.

Read our review of the best instant photo printer .

Sonos Move Speaker (First Gen) — Our Pick Costco deal price: $280, Costco membership required ; street price: $400

What we like: Our pick for a portable smart speaker for your outdoor space. High-quality and wireless. Sounds good. All the benefits of the Sonos system. Supports Airplay 2, Amazon Alexa, and Google Assistant. IP56-rated water resistance. Solid 10-hour battery life.

Other things to know: Audio is notably directional, with a dead spot directly behind it. The charging base is not waterproof. No stereo separation. Sonos released the second generation model last year. Available in black for Costco members.

Read our review of the best smart devices for making the most of your backyard and garden .

Apple MacBook Air (M1, 2020, 8 GB RAM, 256 GB SSD) — Budget Pick (For Now) Walmart deal price: $700 in select colors ; street price: $750

What we like: Our soon-to-be-replaced budget pick from our guide to the best MacBooks. Still enough memory and storage for most day-to-day work. Approximately 14-hour battery life. Lightweight with solid construction, a comfortable keyboard, and industry-leading customer support. M1 processor still good enough for everyday tasks, non-power users.

Other things to know: M3 processor is finally out and the M1 is starting to show its age after three years. Still a worthwhile everyday laptop, but we will likely see better deals in the future as M3 adoption becomes more widespread. Comes with 8 GB RAM, 256 GB SSD, 13.3-inch Retina display, one 3.5mm jack, and two Thunderbolt 3 ports. Available in select finishes. $30 below the previous lowest price.

Vantrue N4 Dash Cam — Top Pick Vantrue deal price: $175 with code WIREVAN4 ; street price: $220

What we like: Our top pick for best dash cam. Crisp, super-high-resolution footage day and night. Front-facing, interior, and rear-facing cameras. 24-hour parked-car monitoring. Compact and relatively unobtrusive. Includes 12-volt car charger.

Other things to know: Performance suffers in temperatures below 14 degrees Fahrenheit. Lacks app connectivity to remotely view and download video. Use code WIREVAN4 for deal from Vantrue. Matches low.

Read our review of the best dash cam .

Google Pixel 8 (128 GB, Unlocked) — Top Pick Amazon deal price: $500 ; street price: $620

What we like: Our top pick for the best Android smartphone offers the best version of Android with guaranteed security updates through fall 2030. Has one of the best smartphone cameras we’ve tested. Excellent build quality. Crisp 6.2-inch OLED display and solid wrap-around aluminum frame.

Other things to know: More expensive than previous models. Videos can often look a little grainy but have good color reproduction. Deal is for an unlocked phone with 128 GB of internal storage.

Read our review of the best Android phones .

Garmin Forerunner 255S GPS Running Watch — Runner-Up Amazon deal price: $250 ; street price: $350

What we like: One of our runner-up picks for the best GPS running watch. One of the most feature-packed, top-performing models we’ve tested. Accurate distance and route tracking. Solid heart-rate monitoring. Multiband GPS.

Other things to know: Decent battery life (about 14 days), but much shorter than that of our top pick. No touchscreen. Not quite as accurate at tracking road runs as our top pick. One-year warranty.

Read our review of the best GPS running watches .

Garmin Forerunner 255S Music GPS Running Watch — Runner-Up Amazon deal price: $300 ; street price: $380

What we like: Our runner-up pick for the best GPS running watch if listening to music is important for your workouts. Still one of the most feature-packed, top-performing GPS running watches we’ve tested. Includes music streaming and storage capabilities.

Other things to know: Accurate distance and route tracking. Solid heart-rate monitoring. Multiband GPS. Decent battery life, about 14 days. Lacks touchscreen. Much shorter battery life than our top pick. In tracking road runs, it’s not quite as accurate as our top pick. One-year warranty.

Cricut Explore 3 Electronic Cutting Machine — Top Pick Amazon deal price: $228 ; street price: $300

What we like: Our top for the best electronic cutting machine. Intuitive and easy-to-use software is great for beginners. Superior selection of images and ready-to-make projects.

Other things to know: Faster than other cutting machines but significantly louder. Doesn’t come with a cutting mat and can only cut materials that are 6 inches or longer. Only the second deal we’ve posted on the Explore 3 so far. After another drop, now down to the lowest price we've seen to date.

Read our review of the best electronic cutting machines .

Brother P-Touch Cube Label Maker — Budget Pick Amazon deal price: $45 ; street price: $60

What we like: Our budget pick for the best label maker. Easy to use. Good for occasional label making. Produces the same high-quality labels and uses the same great design suite as our top pick.

Other things to know: A stripped-down version of our top pick. Lacks automatic cutter and built-in rechargeable battery. Recurring deal price, but rarely drops lower.

Read our review of the best label makers .

Aura Carver Mat Digital Photo Frame — Pick Variant Amazon deal price: $143 in select colors ; street price: $180

What we like: A variant of our top digital photo frame pick. Offers easy setup, vivid display, pleasing design, and near-zero pillarboxing. Loading photos and videos remotely is a breeze. Features two frame layers that give the look of a mat picture frame.

Other things to know: The 16:10 aspect ratio isn’t our preferred 4:3. Free Aura app works with iOS and Android phones. Available in black or clay color with white mat.

Read our review of the best digital photo frame .

Smartish Gripmunk with MagSafe for iPhone 15 Case — Our Pick Amazon deal price: $16 in select colors ; street price: $20

What we like: Our pick for the best basic phone case for the iPhone 15. A one-piece case made of soft, rubber-like plastic (TPU, or thermoplastic polyurethane), and provides more than adequate coverage and protection. Easy to install but won't stretch out. Affordable but doesn’t feel cheap.

Other things to know: Has been our pick for the best basic iPhone case for years now and continues to maintain that spot, but is a plain case with not much going on. Select colors on sale. The iPhone 15 Plus, Pro, and Pro Max versions are also on sale in a few colors.

Read our review of the best iPhone cases .

Close up of a person wearing our pick for the best leggings with pockets, the IUGA High-Waist Yoga Pants leggings.

IUGA High-Waist Yoga Pants — Our Pick Amazon deal price: $20 in select colors and sizes ; street price: $24

What we like: Our cheap and surprisingly soft leggings pick. Fit and felt better than other leggings we tried in the same price range. Soft, comfy, and generally flattering cut. Gentle but supportive compression. Functional, convenient pockets.

Other things to know: Some may find the one-inch elastic waistband to be constricting. A little loose at the ankle for some testers. We found the fabric lost its shape a bit over time. Deal price is for select colors and sizes.

Read our review of the best leggings .

Sungait Vintage Round Sunglasses (Polarized Lenses, 2-Pack) — Our Pick Amazon deal price: $15 with clipped on-page coupon for a 2-pack in select colors ; street price: $18

What we like: Our cheap round sunglasses pick. Also recommended in gift guides and outdoor guides. Comfortable and sturdy. Complements a variety of face shapes. Keyhole nose bridge. Mini screwdriver provided with purchase. Logo-free and unisex.

Other things to know: Lifetime warranty (rare at this price). Some Sungait pairs sold on Amazon don't have polarized lenses, double-check before buying. Deal is for a 2-pack in select frame colors. Clip on-page coupon to get deal for some colors.

Read our review of the best cheap sunglasses .

Lunya Washable Silk Tulip Back Set — Gift Pick Lunya deal price: $150 with code FOR-WCSLEEP ; street price: $188

What we like: A slinky silk pajama set we recommend in our guide to the best gifts for girlfriends (that are great for anyone). An everyday indulgence. Can be hand- or machine-washed, unlike many silk pajamas. Cut of top (with its criss-crossed back) is flattering on many different body types.

Other things to know: 100% Mulberry silk. More playful silhouette than some of our pajama picks. Abbreviated crop top exposes some midriff. Available in a variety of colors and sizes. Use code FOR-WCSLEEP .

Read our review of the best gifts for girlfriends .

A.L.C. Renzo Pleated Midi Dress — Our Pick Bloomingdale's deal price: $446 ; street price: $595

What we like: Our pick for the ultimate little black cocktail dress. Delicate pleats, a high neck, and exposed shoulders make this a refined choice for an evening out. Breathable polyester felt softer and more luxurious than many silk dresses we tested.

Other things to know: The priciest dress we recommend, but worth the investment. Designed to hit midcalf, but may hit closer to ankle level if under 5-foot-5. Sold in sizes 00 to 14.

Read our review of the best little black dresses .

Paige Cindy Jeans — Top Pick Paige deal price: $164 with code PAIGEFAMILY24 ; street price: $219

What we like: Our decadently soft and stretchy women's jeans pick. Straight-leg, high-rise style, fitted through the waist and hips. Body-conscious yet totally lounge-able. Testers of different sizes and shapes found these jeans flattering and wildly comfortable.

Other things to know: Stretchier denim tends to (you guessed it) stretch out over time. 28-inch inseam may be too short for some. Destroyed hem isn't for everyone. Run slightly large; we recommend sizing down if between sizes. Use code PAIGEFAMILY2024 . Doesn't quite compare to deals we saw over Black Friday, but still a good value.

Read our review of the best women's jeans .

Nike Blazer Mid '77 Vintage Sneakers (Men's) — Staff Favorite Nike deal price: $72 in select colors with code SPRING, free Nike membership required ; street price: $105

What we like: A staff favorite alternative to Nike Air Force 1s. Versatile mid-height sneakers with a vintage feel. Sleek, leather upper with suede details. Solid rubber sole provides strong foot support and traction.

Other things to know: Free Nike membership required to use coupon code SPRING and for free shipping. Several colors are on sale, but the tan option offers a wide range of sizes for a neutral color.

Read our ode to the Nike Blazer Mid '77 Vintage Sneakers .

Crocs Dylan Clog — Gift Pick Amazon deal price: $33 in select colors ; street price: $55

What we like: A more sophisticated pair of Crocs we recommend in several of our gift guides. Belies the brand's ugly reputation. Textured to look like full-grain leather. Clean profile with a notably unperforated toe box.

Other things to know: Easy to wash thanks to proprietary resin construction. On sale in select colors in women's and men's sizes. New low.

Read our review of the best gifts for boyfriends (that are great for anyone) .

Other things to know: Fabric shell is abrasive and may make some sweaters and blouses pill. The weave also attracts pet hair like a magnet. Available in ranger green. Lowest price we've seen so far.

Bonobos Soft Everyday Tee (pack of two) — Our Pick Bonobos deal price: $50 when you add two to cart ; street price: $70

What we like: Our boxier, more relaxed white T-shirt pick. Extremely soft with some stretch and sturdily constructed. Especially loved by our plus-size testers for quality, fit, and comfort. 100% cotton.

Other things to know: Generous fit may not work for people with smaller frames. Available in a variety of colors and sizes. Choose between crew neck or V-neck and slim or standard cut. Add two to your cart to get the deal price.

Read our review of the best men’s white T-shirts .

What we like: A casual leather boot loved by Wirecutter staff and many others around the world. Pull-on, ankle-high, with an elastic panel. Comfortable, durable rubber sole. Uppers made of extra-thick leather. Ergonomically engineered to reduce fatigue. They can last for years with daily wear.

Etsfmoa Unisex Beanie with Light — Gift Pick Amazon deal price: $13 in select colors ; street price: $15

What we like: A combination beanie and headlamp recommended in multiple Wirecutter gift guides. A bright, rechargeable light embedded in a soft, warm hat. Light charges in about two hours via USB port and has three brightness settings. Beanie comes in 24 different colors and patterns.

Other things to know: Great for walking the dog after dark or staying hands-free during house projects (also makes a good Father’s Day gift). The black and gray hats, the typical colors we see on sale, are both down to $13.

Read our review of the best gifts under $25 .

Idegg No Show Socks (Unisex, Six Pairs) — Our Pick Amazon deal price: $12 for a 6-pack in select colors and sizes ; street price: $15

What we like: Our thin and inexpensive no-show socks pick. Made from a lightweight and breathable cotton blend. Terrific value, especially when on sale. Great for wearing with snug-fitting shoes. Won’t slip or bunch.

Other things to know: Thin material doesn’t provide much padding or support. Not as soft and smooth as our other no-show sock picks. Deal is for a medium six-pack combination of black, gray, and whites (add $1 for large).

Read our review of the best no-show socks .

The Dyson Supersonic Hair Dryer, our upgrade pick, next to its three attachments.

Dyson Supersonic Hair Dryer (Refurbished) — Upgrade Pick Walmart deal price: $220 ; street price: $300

What we like: Our longtime upgrade hair dryer pick. Unique design makes for an extraordinarily pleasant user experience. No vibration whatsoever. Simple-to-switch magnetic attachments make styling a breeze. Consistently our favorite hair dryer to use since we first tested it in 2016.

Other things to know: Pricey, and doesn’t make your hair look any different than cheaper options. Manufacturer refurbished. On sale in select finishes. One-year Dyson warranty and 30-day returns. Doesn't match short-lived $207 deal we recently saw, but still a good value.

Read our review of the best hair dryer .

Govee Wi-Fi Water Sensors (2-Pack) — Another Good Option Amazon deal price: $20 ; street price: $40

What we like: Another good, budget-friendly option in our guide to the best smart water-leak detector. Compatible with Alexa. Loud siren. Uses a plug-in Wi-Fi hub to function.

Other things to know: Alarm can only be silenced on sensor itself. No alert if device loses Wi-Fi and/or power connection. Deal includes a two-pack of sensors and one hub. Clip the on-page coupon.

Read our review of the best smart water-leak detector .

Eufy P2 Pro Smart Scale — Runner-Up Amazon deal price: $41 ; street price: $75

What we like: Our runner-up pick for the best smart scale is accurate and consistent. Exceptionally water-resistant. Has a robust and easy-to-use app. Connects via Bluetooth or Wi-Fi. Compatible with Apple Health, Google Fit, and Fitbit.

Other things to know: Typically double the price of our top pick and is slightly smaller and less consistently accurate. Requires more frequent battery replacement than our top pick.

Read our review of the best smart scale .

Schlage Encode Smart WiFi Deadbolt (Century Trim) — Top Pick Amazon deal price: $183 ; street price: $230

What we like: Our top pick for the best smart deadbolt lock. A low-profile, no-fuss lock with a foolproof keypad and the ability to connect to Wi-Fi without requiring a plug-in adapter. Particularly easy to install, and using it is intuitive. Battery life has been excellent, lasting six months or more at a time.

Other things to know: Compatible with Amazon Alexa and Google Home, but not Apple Home. Relatively loud and slow, which sometimes leads to accidental lock jams when people open the door while it’s still retracting the bolt. The Schlage app is not great. Deal for the century trim in the satin nickel finish. Clip the on-page coupon.

Read our review of the best smart locks .

Crest 3D Whitestrips Professional Effects Kit (22 Treatments) — Staff Favorite Amazon deal price: $30 ; street price: $46

What we like: An at-home teeth-whitening solution that is a staff favorite. Low-tech, effective, and very affordable. No dentist visits required. Comes with 10 days' worth of Professional Effects treatments and two 1-Hour Express treatments.

Other things to know: Slower than professional treatments—results take several sessions over the course of a few weeks. Not as good as the typical Costco Crest 3D Whitestrips deals we see, but a good discount for those without a Costco membership.

Read our ode to Crest 3D Whitestrips .

Yamazaki Home Tower Shoe Rack — Upgrade Pick Amazon deal price: $94 in select colors ; street price: $115

What we like: The investment-worthy upgrade pick from our guide to the best shoe racks. Slim enough to fit in a closet, but sleek enough for an entryway. Stores up to 24 pairs of shoes. Steel and wood veneer. Top could double as an entryway shelf. Two hooks on the side for keys or an umbrella.

Other things to know: The rack assembles quickly but will require using a screwdriver. Only holds adult-sized shoes; tiny shoes will tumble through. Only on sale in white.

Read our review of the best shoe rack .

Sabai The Essential Sofa — Our Pick Sabai deal price: $1,408 shipped ; street price: $1,640

What we like: Our favorite eco-friendly three-seater sofa pick. Cushions remain plump and unworn after years of use. Available in multiple configurations, fabrics, and colors. Many components are replaceable and recyclable.

Other things to know: Low back and shallow seating may not be the comfiest for lounging. Deal price reflects sample configuration with included shipping cost. The Essential Sofa is made to order and ships within two to four weeks.

Read our review of the best the best sofas you can buy online .

Threshold White Waffle Weave Shower Curtain — Our Pick Target deal price: $20 with store pickup (free shipping for RedCard holders) ; street price: $25

What we like: Our longtime pick for the best classic waffle-weave shower curtain. Simple, timeless, and durable. Easy to clean. Neutral design will complement just about any bathroom.

Other things to know: Available in white or gray. Select store pickup or make a combined order of $35 or more to avoid shipping fees. RedCard holders get free shipping and save an extra 5%.

Read our review of the best shower curtains .

Apotheke Charcoal Candle — Our Pick Bloomingdale's deal price: $33 in cart ; street price: $44

What we like: Our elegant, monochromatic pick for a warm and woodsy scented candle. Notes of charred sandalwood and cedar after a heavy rain. Fragrant but not heavy or overpowering. Sleek matte-black vessel is as stylish as it is adept at hiding soot streaks, fingerprints, and other smudges.

Other things to know: About 70-hour burn time. We wish it came with a lid or dust cover. Jar is reusable, but the company name is printed on the glass, so it’s impossible to remove. Price reflects in cart. We've seen better deals for the 3-wick size, but this is a solid deal for this size.

Read our review of our favorite scented candles .

Lutron Caséta Wireless In-Wall Smart Dimmer Switch (2-Count Kit) — Another Good Option Amazon deal price: $136 ; street price: $170

What we like: A good smart light dimmer switch if you don’t have great Wi-Fi. Easy-to-use keypad for dimming and powering lights manually. Easy to install. Works reliably with many smart-home devices. A pick when we started this guide, and we will continue to sing its praises.

Other things to know: Includes two switches, required Lutron Caséta Smart Bridge, and two wireless remotes. Uses ClearConnect wireless. Neutral wire required (which may be an issue in homes with old wiring).

Read our review of the best in-wall smart light switch and dimmer .

Akron Street Mysa Bed (Queen) — Our Pick Akron Street deal price: $1,166 in cart with code TOTHETRADE ; street price: $1,295

What we like: Our pick for a no-frills, all-oak modern bed. Supremely simple, clean aesthetic with a low profile. Solid white oak construction. Supported by 15 acacia pine slats. Easy-to-assemble. Built to last. Works especially well with tall mattresses and small bedrooms.

Other things to know: Has 11.5-inch height, with an under-bed clearance of 6.8 inches. No warranty. Price reflects in cart or use code TOTHETRADE . Standard shipping is free.

Read our review of the best modern bed frames to dress up your bedroom .

Homeasy Fabric Shaver — Our Pick Amazon deal price: $22 ; street price: $25

What we like: Our favorite electric fabric shaver. Features six sharp blades, a wide head, and intuitive controls. Very effective at getting rid of fuzz during our tests. Quiet and comfortable to hold. USB-rechargeable.

Other thing to know: Three to six hour battery life on a full charge. Includes two replacement blades. About $2 higher than the lowest sale price we've seen, but still a good value.

Read our review of the best laundry aides to help your clothes look better and last longer .

Revival Rugs - 25% Off Jute Area Rugs — Our Pick Revival deal price: $219 with code WCJUTE25 ; street price: $290

What we like: Our pick for a natural-fiber rug that comes in several colors. Flexible and finely woven, giving it a more delicate, flat appearance. Feels soft underfoot and barely sheds. Easy to vacuum and lightweight enough to shake out outside.

Other things to know: Best for dining rooms, kitchens, hallways, or layering in living rooms or bedrooms. Spot-clean only, as jute is very susceptible to water and humidity. Use code WCJUTE25 to save 25% on all jute rugs. Deal price is for the 5-by-8-foot size, but pricing varies depending on total cost; no minimum spend.

Read our review of the best area rugs .

Waterpik Aquarius Water Flosser — Runner-Up Amazon deal price: $58 ; street price: $80

What we like: Our runner-up pick for the best water flosser is just as effective as our top pick and comes with a similar three-year warranty. Easy to use and clean. Ten pressure settings, including a “massage” setting that emits pulses of water.

Other things to know: This is a corded model and requires a power outlet during use. Takes up more counter space than our top pick. Recurring deal price; $10 more than the deal we saw during Cyber Week.

Read our review of the best water flossers .

Amazon Basics 8-Sheet High-Security Micro-Cut Shredder with Pullout Basket — Top Pick Amazon deal price: $62 ; street price: $69

What we like: Our top pick for the best paper shredder. This high-security shredder offers the best balance of ease of use, sheet capacity, and price. It can run jam-free for nearly 10 minutes without overheating. Micro-cuts documents and has a separate CD/DVD shredding slot.

Other things to know: Slightly bigger than eight-sheet shredders with lift-up tops. The overload LED indicator didn't work like we expected it would. We recommend unplugging any shredder when not in use.

Read our review of the best paper shredders .

A cat lying on a blanket next to the Black+Decker Pet Hair Remover.

Black+Decker Pet Hair Remover — Staff Favorite Walmart deal price: $10 with store pickup ; street price: $16

What we like: A lint roller for pet hair recommended by the Wirecutter staff. Reusable and tape-free. Effectively catches stray fur on fabric surfaces. The brush is made of velvet and silicone that grips the fur and traps it in a bin behind the roller. Works great on bedding.

Other things to know: Roll it back and forth on your bedding, couch, or clothing to pick up hair, and press the button to empty it once the bin is full. Opt for store pickup to avoid shipping fees or add to a combined order over $35 for free shipping.

Read our review of the best pet hair removers .

PetKit Eversweet Solo 2 Water Fountain — Upcoming Pick Amazon deal price: $40 ; street price: $50

What we like: An upcoming pick in our pet water fountain guide. Only five main parts to keep track of. The easiest fountain to take apart and clean in testing. Nearly silent wireless pump has a convenient auto shut-off feature, so it doesn’t burn out when the water level gets too low.

Other things to know: Also comes with a companion app, which lets you set up cleaning and refill reminders, but it has a limited feature set.

Read our review of the best water fountain for cats and dogs .

PetFusion Jumbo Cat Scratcher Lounge — Upgrade Pick Amazon deal price: $68 ; street price: $80

What we like: Our pick for the best cat bed for multicat households, this doubles as a scratcher, too. Both the largest bed we tested and the nicest scratcher we found. Big enough to sleep four cats comfortably. You can flip it over and use the underside when the top side shows wear and tear, prolonging its lifespan.

Other things to know: Large, about 39 by 14 inches. Cardboard may be too densely packed for some cats to comfortably scratch.

Read our review of the best cat beds (according to our cats) .

Editor’s note: The photo at the top of this article may reflect previous deals that are no longer valid and have been removed. Check out our Deals page for more great discounts on Wirecutter’s expert-approved picks.

This article was edited by Janie Campbell and Nathan Burrow.

Meet your guide

Wirecutter Staff

light travel fastest in vacuum

Solar eclipse 2024: A traveller’s guide to the best places to be when the light goes out

O n 8 April 2024, a total solar eclipse will sweep across North America , providing an astronomical experience in many alluring locations.

Only a tiny proportion of humanity has ever witnessed a total eclipse – but tens of millions of people will be able to experience one as the “path of totality” sweeps from the Pacific to the Atlantic during the course of that magical Monday.

Here’s what you need to know about why you should see it and where to be.

What happens during a total solar eclipse?

The greatest show on earth comes courtesy of the lifeless moon. Normally the orbiting lunar lump merely provides earth with tides, moonlight and somewhere to aim space rockets. But roughly once a year the natural satellite aligns with the sun and, thanks to a geometric miracle, blots out the hub of the solar system to create a total eclipse.

“Even though the moon is 400 times smaller than the sun, it’s also about 400 times closer to earth than the sun is,” says Nasa. “This means that from earth, the moon and the sun appear to be roughly the same size in the sky.”

A narrow band marking the “path of totality” carves an arc of darkness across the surface of our planet. If you are somewhere on that line at the predicted time, and you have clear skies, then the experience will become a lifelong memory.

The closer you are to the centre of the path of totality, the longer the total eclipse will last. The astronomer Dr John Mason, who has guided dozens of eclipse trips (and will be doing so again in 2024), says: “People down in southwest Texas will get about four minutes 20 seconds, and that reduces to about three minutes 20 seconds up in the northeast. That’s a pretty good, long total eclipse.”

What’s so good about seeing an eclipse?

In the days leading up to the eclipse, locations in the path of totality acquire something of a carnival atmosphere as astronomical tourists converge in excited anticipation.

On the day, the cosmological performance begins with a warm-up lasting more than an hour, during which the moon steadily nibbles away at the surface of the sun.

Suddenly, you experience totality. The stars and planets appear in the middle of the day. The air chills.

To testify to the heavenly fit between our two most familiar heavenly bodies, faint diamonds known as Baily’s beads peek out from behind the moon. They actually comprise light from the sun slipping through lunar valleys.

A sight to behold – so long as you can see the moon blotting out the sun and appreciate the mathematical perfection of nature in our corner of the galaxy.

Eclipses are entirely predictable: we know the stripes that the next few dozen will paint upon the surface of the Earth. But the weather is not. Cloud cover, which blighted the Cornwall eclipse in 1999, downgrades a cosmological marvel to an eerie daytime gloom.

Almost as predictable as the eclipse is that traffic towards the path of totality will be heavy on the morning of 8 April 2024.

Accommodation rates are astronomical: even humdrum motel rooms in Niagara are selling for C$600 (£350) for the night of 7-8 April 2024.

Where will the great American eclipse 2024 be visible?

The path of totality makes landfall from the Pacific at Mazatlan on Mexico’s Pacific Coast and sweeps northeastwards to reach the US-Mexican border at Piedras Negras.

Big Texan cities – San Antonio, Austin and Dallas – are on the extremes of the path of totality; many citizens are likely to drive to locations near the centre of the line.

Arkansas will be an attractive place to see the eclipse , with both Texarkana (on the border with Texas) and Little Rock within the path of totality.

In the Midwest, Indianapolis and Cleveland share the distinction of being fairly central in the path of totality. In upstate New York, Buffalo and Niagara Falls could be extremely attractive – though prone in early April to cloudy skies.

In Canada , Montreal is just touched by the path of totality. The line then reverts to the US, passing across northern Maine – which would be a superb location were the weather good. Then back to Canada’s Maritime Provinces, with New Brunswick, Prince Edward Island and Newfoundland all in the line of darkness.

I’d rather be in a more exciting city – will they get a partial eclipse?

Boston, New York and Chicago are among the big cities that will see a sizeable chunk of the sun blotted out. Viewer as far apart as Alaska and the far north of Colombia and the Caribbean will, if skies are clear and they use the correct eye protection, see a partial eclipse. But there is nothing to compare with a total eclipse.

Eclipse guru Dr Mason sums up the difference between a 99 per cent partial eclipse and a total eclipse as far apart as “a peck on the cheek and a night of passion”.

“There will be people who will look at the map and say, ‘I live in Cincinnati or I live in Columbus [Ohio] and I’m just outside the zone of totality. But I’m going to get a 99 and a half per cent-plus eclipse, so maybe I won’t bother to travel’.

“What they don’t realise is there an enormous difference between 99 per cent and 100 per cent. And there’s a range of phenomena that they won’t see if they put up with 99 per cent.”

You must use special eclipse safety glasses or viewers when viewing a partial eclipse or during the partial phases of a total solar eclipse.

Where should I be for the total experience?

There are no guarantees of clear skies: all you can do is play the odds based on the record of cloud cover for the corresponding date in previous years.

Dr Mason says the average expected cloud cover amounts increase from around 40-45 per cent on the Mexico/Texas border to over 80 per cent in Maine, New Brunswick and Newfoundland.

Three particularly tempting locations:

  • Southern Texas , close to San Antonio or Austin. Besides clear skies being more likely than not, access is easy with direct flights to Austin. Importantly there is much to explore in the region before and after the eclipse, from Big Bend National Park on the Rio Grande to Space Center Houston – an excellent place to continue the cosmological theme.
  • Northern Arkansas , a picturesque part of the state, with the added attraction of Memphis just a couple of hours away.
  • Niagara Falls : the dramatic border between the US and Canada could be an eclipse washout due to clouds. But the natural surroundings are impeccable – and there is plenty of accommodation, which will avoid the risk of being caught in severe traffic congestion on the freeways from Toronto and locations in New York State.

Will I be able to see a partial eclipse from the UK?

No. The eclipse ends with the sunset in the eastern Atlantic, off the coast of Portugal, before it reaches the UK and Ireland.

When are the next total solar eclipses?

Summer 2026 – Wednesday 12 August, to be precise – should bring a spectacular eclipse visible in northern Spain at the height of the European holiday season. The path of totality begins in the Arctic and crosses Greenland and Iceland before arriving in the northern half of Spain. The stripe of darkness will traverse the great cities of Bilbao, Zaragoza and Valencia in mainland Spain before arriving in Palma de Mallorca.

The following summer (2 August 2027), the southern tip of mainland Spain is in the path of totality for an eclipse that will sweep across North Africa and the Arabian peninsula : going east from the Strait of Gibraltar, it will encompass Morocco, Algeria, Tunisia, Libya, Egypt, the northeasternmost corner of Sudan, Saudi Arabia and Yemen.

Just under 12 months later, on 22 July 2028, Outback Australia will be the place to be. A total eclipse will make landfall in northern Western Australia, sweep across the Northern Territory and part of southwest Queensland – then clean across New South Wales, with Sydney in the middle of the path of totality.

Winter cloud cover could disrupt the experience in Australia’s largest city – and is very likely in the southern portion of New Zealand’s South Island where the eclipse reaches a finale.

Australia also features in the cosmological plans on 25 November 2030. This is early summer in the southern hemisphere, and likely to be good conditions for viewing in Namibia, Botswana and South Africa (Durban is on the path of totality) as well as South Australia.

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COMMENTS

  1. How fast does light travel?

    The speed of light in a vacuum is 186,282 miles per second (299,792 kilometers per second), and in theory nothing can travel faster than light.

  2. Speed of light

    The speed of light in vacuum, commonly denoted c, is a universal physical constant that is exactly equal to 299,792,458 metres per second (approximately 300,000 kilometres per second; 186,000 miles per second; 671 million miles per hour). According to the special theory of relativity, c is the upper limit for the speed at which conventional matter or energy (and thus any signal carrying ...

  3. Physics Explained: Here's Why The Speed of Light Is The ...

    Today the speed of light, or c as it's commonly known, is considered the cornerstone of special relativity - unlike space and time, the speed of light is constant, independent of the observer. What's more, this constant underpins much of what we understand about the Universe. It matches the speed of a gravitational wave, and yes, it's the ...

  4. Speed of light: How fast light travels, explained simply and clearly

    On one hand, the speed of light is just a number: 299,792,458 meters per second. And on the other, it's one of the most important constants that appears in nature and defines the relationship of ...

  5. What is light, and how can it travel in a vacuum forever in all

    How can light (or electromagnetic radiation) travel through a vacuum when there is nothing there to act as a medium, and do so forever in all directions? For example the light coming from a star millions of light years away. Light is observed as traveling at velocity v=c, according to the second postulate of special relativity. But according to ...

  6. What is the speed of light?

    Nothing can travel faster than light — not even our best spacecraft — according to the laws of physics. ... In a vacuum, such as outer space, light travels at a constant speed of approximately ...

  7. Can anything travel faster than the speed of light?

    When light travels through a vacuum, however, the same is not true. "All light is a type of electromagnetic wave, and they all have the same speed in a vacuum (3 x 10^8 meters per second).

  8. Speed of light

    Speed of light, speed at which light waves propagate through different materials. In a vacuum, the speed of light is 299,792,458 meters per second. The speed of light is considered a fundamental constant of nature. Its significance is far broader than its role in describing a property of electromagnetic waves.

  9. How Does Light Travel?

    Light actually "slows down" every time it has to travel through anything but a vacuum. Look up Cherenkov radiation to see what happens when light initially travels faster than it can through a ...

  10. How do we know that light can travel through a vacuum?

    The speed of light in vacuum c is not measured. It has an exact fixed value when given in standard units. Since 1983 the meter has been defined by international agreement as the length of the path traveled by light in vacuum during a time interval of 1/299792458 of a second. This makes the speed of light exactly 299,792.458 km/s.

  11. 2.1: The Wave Nature of Light

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    Callie: "Right, radio waves are light waves. The radio is really detecting light waves in the radio portion of the spectrum and converting them into sound waves that we can hear." ... Which colors of light do you think travel fastest through the vacuum of empty space? 1. Rank the wavebands: gamma-ray, x-ray, optical, and radio, according to ...

  14. Particles Found to Travel Faster Than Speed of Light

    The idea that nothing can travel faster than light in a vacuum is the cornerstone of Albert Einstein's special theory of relativity, which itself forms the foundation of modern physics.

  15. Refraction of Light

    The Speed of Light in Various Medium. Light is an electromagnetic wave that travels fastest in the vacuum of space, slower in gas, and slowest in a solid because of density. Review our other lesson about speed of an electromagnetic wave and density following this link. Light has a different speed in different translucent solids.

  16. 5.1.1: Speeds of Different Types of Waves

    5.1.1: Speeds of Different Types of Waves. The speed of a wave is fixed by the type of wave and the physical properties of the medium in which it travels. An exception is electromagnetic waves which can travel through a vacuum. For most substances the material will vibrate obeying a Hooke's law force as a wave passes through it and the speed ...

  17. Which electromagnetic waves travel the fastest in a vacuum?

    Homework Statement Which electromagnetic waves travel the fastest in a vacuum? a. Violet light. b. Red light. c. Ultraviolet light. d. Red, violet, and ultraviolet light travel at the same speed. ok so i thought that in a vacuum every wave traveled at the same velocity, so only the frequency could vary. I know the ultraviolet light has the highest frequency, so i was assuming d was the correct ...

  18. Why does light travel faster in vacuum

    In a vacuum, there are no atoms or molecules to interact with, so light can travel at its maximum speed of 299,792,458 meters per second. This same speed is known as the speed of light, and it is the fastest speed that anything can travel in the universe.

  19. Does light travel faster in glass or vacuum?

    Refractive Index Light's speed is reduced when it travels through a medium due to the interaction of photons with electrons. Typically, higher electron densities in a material result in lower velocities. This is why light travels fast in glass, faster in water, and fastest in a vacuum.

  20. Why does light travel faster in vacuum?

    Solution. Light travels faster in vacuum than any other medium. This is because there is no obstruction in vacuum for the propagation of light and thus, the refractive index of vacuum is the lowest. Show the speed of light in vacuum is the maximum. Was this answer helpful?

  21. Why does Light Travel Faster in a Vacuum

    The rapidity of light is a speed limit on the whole thing in our universe. Light itinerant through a vacuum transfers at accurately 299,792,458 meters (983,571,056 feet) per second. That's around 186,282 miles per second a general constant known in calculations and in shorthand as "c," or the speed of light.

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  29. Solar eclipse 2024: A traveller's guide to the best places to ...

    But I'm going to get a 99 and a half per cent-plus eclipse, so maybe I won't bother to travel'. "What they don't realise is there an enormous difference between 99 per cent and 100 per cent.