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Sunrise viewed from the space shuttle

Light

by Chris Woodford. Last updated: July 4, 2013.

Were you ever scared of the dark? It's not surprising if you were, or if you still are today, because humans are creatures of the light, deeply programmed through millions of years of history to avoid the dark dangers of the night. Light is vitally important to us, but we don't always take the trouble to understand it. Why does it make some things appear to be different colors from others? Does it travel as particles or as waves? Why does it move so quickly? Let's take a closer look at some of these questions—let's shed some light on light!

Photo: Light floods into our world from the Sun. Photo taken from the Space Shuttle by courtesy of NASA Johnson Space Center: Earth Sciences and Image Analysis (NASA-JSC-ES&IA).

What is light?

When we're very young, we have a very simple idea about light: the world is either light or dark and we can change from one to the other just by flicking a switch on the wall. But we soon learn that light is more complex than this.

Light arrives on our planet after a speedy trip from the Sun, 149 million km (93 million miles away). Light travels at 186,000 miles (300,000 km) per second, so the light you're seeing now was still tucked away in the Sun about eight minutes ago. Put it another way, light takes roughly twice as long to get from the Sun to Earth as it does to make a cup of coffee!

Light is a kind of energy

Supernova

But why does light make this journey at all? As you probably know, the Sun is a nuclear fireball spewing energy in all directions. The light that we see it simply the one part of the energy that the Sun makes that our eyes can detect. When light travels between two places (from the Sun to the Earth or from a flashlight to the sidewalk in front of you on a dark night), energy makes a journey between those two points. The energy travels in the form of waves (similar to the waves on the sea but about 100 million times smaller)—a vibrating pattern of electricity and magnetism that we call electromagnetic energy. If our eyes could see electricity and magnetism, we might see each ray of light as a wave of electricity vibrating in one direction and a wave of magnetism vibrating at right angles to it. These two waves would travel in step and at the speed of light.

Picture: Light energy likes to travel outwards! This bubble-shaped explosion of gas and dust is 14 light-years wide and is expanding at 4 million miles per hour (2,000 kilometers per second). It's the remnant of a supernova (exploding star). Photo courtesy of NASA Jet Propulsion Laboratory (NASA-JPL).

Is light a particle or a wave?

Isaac Newton

For hundreds of years, scientists have argued over whether light is really a wave at all. Back in the 17th century, the brilliant English scientist Sir Isaac Newton (1642–1727)—one of the first people to study the matter in detail—thought light was a stream of "corpuscles" or particles. But his great rival, a no-less-brilliant Dutchman named Christiaan Huygens (1629–1695), was quite adamant that light was made up of waves.

Photo: Isaac Newton argued that light was a stream of particles. Picture courtesy of US Library of Congress.

Thus began a controversy that still rumbles on today—and it's easy to see why. In some ways, light behaves just like a wave: light reflects off a mirror, for example, in exactly the same way that waves crashing in from the sea "reflect" off sea walls and go back out again. In other ways, light behaves much more like a stream of particles—like bullets firing in rapid succession from a gun. During the 20th century, physicists came to believe that light could be both a particle and a wave at the same time. (This idea sounds quite simple, but goes by the rather complex name of wave-particle duality.)

The real answer to this problem is more a matter of philosophy and psychology than physics. Our understanding of the world is based on the way our eyes and brains interpret it. Sometimes it seems to us that light is behaving like a wave; sometimes it seems like light is a stream of particles. We have two mental pigeonholes and light doesn't quite fit into either of them. It's like the glass slipper that doesn't fit either of the ugly sisters (particle or wave). We can pretend it nearly fits both of them, some of the time. But in truth, light is simply what it is—a form of energy that doesn't neatly match our mental scheme of how things should be. One day, someone will come up with a better way of describing and explaining it that makes perfect sense in all situations.

How light behaves

Light waves (let's assume they are indeed waves for now) behave in four particularly interesting and useful ways that we describe as reflection, refraction, diffraction, and interference.

Reflection

Solar mirror

The most obvious thing about light is that it will reflect off things. The only reason we can see the things around us is that light, either from the Sun or from something like an electric lamp here on Earth, reflects off them into our eyes. Cut off the source of the light or stop it from reaching your eyes and those objects disappear. They don't cease to exist, but you can no longer see them.

Photo: Now that's what I call a mirror! A solar mirror used to gather and focus energy from the Sun. Picture by courtesy of NASA Glenn Research Center (NASA-GRC).

Reflection can happen in two quite different ways. If you have a smooth, highly polished surface and you shine a narrow beam of light at it, you get a narrow beam of light reflected back off it. This is called specular reflection and it's what happens if you shine a flashlight or laser into a mirror: you get a well-defined beam of light bouncing back towards you. Most objects aren't smooth and highly polished: they're quite rough. So, when you shine light onto them, it's scattered all over the place. This is called diffuse reflection and it's how we see most objects around us as they scatter the light falling on them.

If you can see your face in something, it's specular reflection; if you can't see your face, it's diffuse reflection. Polish up a teaspoon and you can see your face quite clearly. But if the spoon is dirty, all the bits of dirt and dust are scattering light in all directions and your face disappears.

Refraction

Light waves travel in straight lines through empty space (a vacuum), but more interesting things happen to them when they travel through other materials—especially when they move from one material to another. That's not unusual: we do the same thing ourselves.

Have you noticed how your body slows down when you try to walk through water? You go racing down the beach at top speed but, as soon as you hit the sea, you slow right down. No matter how hard you try, you cannot run as quickly through water as through air. The dense liquid is harder to push out of the way, so it slows you down. Exactly the same thing happens to light if you shine it into water, glass, plastic or another more dense material: it slows down quite dramatically. This tends to make light waves bend—something we usually call refraction.

How refraction works

Refraction of laser beams inside crystals
Photo: Laser beams bending (refracting) through a crystal. Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).

You've probably noticed that water can bend light. You can see this for yourself by putting a straw in a glass of water. Notice how the straw appears to kink at the point where the water meets the air above it. The bending happens not in the water itself but at the junction of the air and the water. You can see the same thing happening in this photo of laser light beams shining between two crystals. As the beams cross the junction, they bend quite noticeably.

Artwork explaining how refraction (the bending of light) happens when light rays slow down

Why does this happen? You may have learned that the speed of light is always the same, but that's only true when light travels in a vacuum. In fact, light travels more slowly in some materials than others. It goes more slowly in water than in air. Or, to put it another way, light slows down when it moves from air to water and it speeds up when it moves from water to air. This is what causes the straw to look bent. Let's look into this a bit more closely.

Imagine a light ray zooming along through the air at an angle to some water. Now imagine that the light ray is actually a line of people swimming along in formation, side-by-side, through the air. The swimmers on one side are going to enter the water more quickly than the swimmers on the other side and, as they do so, they are going to slow down—because people move more slowly in water than in air. That means the whole line is going to start slowing down, beginning with the swimmers at one side and ending with the swimmers on the other side some time later. That's going to cause the entire line to bend at an angle. This is exactly how light behaves when it enters water—and why water makes a straw look bent.

Refraction is amazingly useful. If you wear eyeglasses, you probably know that the lenses they contain are curved-shape pieces of glass or plastic that bend (refract) the light from the things you're looking at. Bending the light makes it seem to come from nearer or further away (depending on the type of lenses you have), which corrects the problem with your sight. To put it another way, your eyeglasses fix your vision by slowing down incoming light so it shifts direction slightly. Binoculars, telescopes, cameras, camcorders, night vision goggles, and many other things with lenses work in exactly the same way (collectively we call these things optical equipment).

Although light normally travels in straight lines, you can make it bend round corners by shooting it down thin glass or plastic pipes called fiber-optic cables. Reflection and refraction are at work inside these "light pipes" to make rays of light follow an unusual path they wouldn't normally take.

Diffraction

We can hear sounds bending round doorways, but we can't see round corners—why is that? Like light, sound travels in the form of waves (they're very different kinds of waves, but the idea of energy traveling in a wave pattern is broadly the same). Sound waves tend to range in size from a few centimeters to a few meters, and they will spread out when they come to an opening that is roughly the same size as they are—something like a doorway, for example. If sound is rushing down a corridor in your general direction and there's a doorway opening onto the room where you're sitting, the sound waves will spread in through the doorway and travel to your ears. The same thing does not happen with light. But light will spread out in an identical way if you shine it on a tiny opening that's of roughly similar size to its wavelength. You may have noticed this effect, which is called diffraction, if you screw your eyes up and look at a streetlight in the dark. As your eyes close, the light seems to spread out in strange stripes as it squeezes through the narrow gaps between your eyelashes. The tighter you close your eyes, the more the light spreads (until it disappears when you close your eyes completely).

Interference

If you stand above a calm pond (or a bath full of water) and dip your finger in (or allow a single drop to drip down to the water surface from a height), you'll see ripples of energy spreading outwards from the point of the impact. If you do this in two different places, the two sets of ripples will move toward one another, crash together, and form a new pattern of ripples called an interference pattern. Light behaves in exactly the same way. If two light sources produce waves of light that travel together and meet up, the waves will interfere with one another where they cross. In some places the crests of waves will reinforce and get bigger, but in other places the crest of one wave will meet the trough of another wave and the two will cancel out.

A rainbow-spectrum of colors on the surface of a soap bubble produced by thin-film interference.

Interference causes effects like the swirling, colored spectrum patterns on the surface of soap bubbles and the similar rainbow effect you can see if you hold a compact disc up to the light. What happens is that two reflected light waves interfere. One light wave reflects from the outer layer of the soap film that wraps around the air bubble, while a second light wave carries on through the soap, only to reflect off its inner layer. The two light waves travel slightly different distances so they get out of step. When they meet up again on the way back out of the bubble, they interfere. This makes the color of the light change in a way that depends on the thickness of the soap bubble. As the soap gradually thins out, the amount of interference changes and the color of the reflected light changes too. Read more about this in our article on thin-film interference.

Photo: Thin-film interference makes the colors you see swirling around on the surface of soap bubbles.

Interference is very colorful, but it has practical uses too. A technique called interferometry can use interfering laser beams to measure incredibly small distances.

Where does light come from?

Welding steel and making sparks

If you've read our article on energy, you'll know that energy is something that doesn't just turn up out of the blue: it has to come from somewhere. There is a fixed amount of energy in the Universe and no process ever creates or destroys energy—it simply turns some of the existing energy into one or more other forms. This idea is a basic law of physics called the conservation of energy and it applies to light as much as anything else. So where then does light comes from? How exactly do you "make" light?

It turns out that light is made inside atoms when they get "excited". That's not excited in the silly, giggling sense of the word, but in a more specialized scientific sense. Think of the electrons inside atoms as a bit like fireflies sitting on a ladder. When an atom absorbs energy, for one reason or another, the electrons get promoted to higher energy levels. Visualize one of the fireflies moving up to a higher rung on the ladder. Unfortunately, the ladder isn't quite so stable with the firefly wobbling about up there, so the fly takes very little persuading to leap back down to where it was before. In so doing, it has to give back the energy it absorbed—and it does that by flashing its tail.

That's pretty much what happens when an atom absorbs energy. An electron inside it jumps to a higher energy level, but makes the atom unstable. As the electron returns to its original level, it gives back the energy as a flash of light called a photon.

Photo: Arc welding gives off light when metals are melted by an electric current. The atoms are getting quite excited here! Picture by Martin Wright, courtesy of US Navy.

How atoms make light

Artwork showing how an atom makes light

Atoms are the tiny particles from which all things are made. Simplified greatly, an atom looks a bit like our solar system, which has the Sun at its center and planets orbiting around it.

Most of the atom's mass is concentrated in the nucleus at the center (red), made from protons and neutrons packed together.

Electrons (blue) are arranged around the nucleus in shells (sometimes called orbitals, or energy levels). The more energy an electron has, the farther it is from the nucleus.

Atoms make light in a three-step process:

  1. They start off in their stable "ground state" with electrons in their normal places.
  2. When they absorb energy, one or more electrons are kicked out farther from the nucleus into higher energy levels. We say the atom is now "excited."
  3. However, an excited atom is unstable and quickly tries to get back to its stable, ground state. So it gives off the excess energy it originally gained as a photon of energy (wiggly line): a packet of light.

How light really works

Once you understand how atoms take in and give out energy, the science of light makes sense in a very interesting new way. Think about mirrors, for example. When you look at a mirror and see your face reflected, what's actually going on? Light (maybe from a window) is hitting your face and bouncing into the mirror. Inside the mirror, atoms of silver (or another very reflective metal) are catching the incoming light energy and becoming excited. That makes them unstable, so they throw out new photons of light that travel back out of the mirror towards you. In effect, the mirror is playing throw and catch with you using photons of light as the balls!

The same idea can help us explain things like photocopiers and solar panels (flat sheets of the chemical element silicon that turn sunlight into electricity). Have you ever wondered why solar panels look black even when they're in full sunlight? That's because they're reflecting back little or none of the light that falls on them and absorbing all the energy instead. (Things that are black absorb light, and reflect little or none, while things that are white reflect virtually all the light that falls on them, and absorb little or none. That's why it's best to wear white clothes on a scorching hot day.) Where does the energy go in a solar panel if it's not reflected? If you shine sunlight onto the solar cells in a solar panel, the atoms of silicon in the cells catch the energy from the sunlight. Then, instead of producing new photons, they produce a flow of electricity instead through what's known as the photoelectric (or photovoltaic) effect. In other words, the incoming solar energy (from the Sun) is converted to outgoing electricity.

Hot light and cold light

What would make an atom absorb energy in the first place? You might give it some energy by heating it up. If you put an iron bar in a blazing fire, the bar would eventually heat up so much that it glowed red hot. What's happening is that you're supplying energy to the iron atoms inside the bar and getting them excited. Their electrons are being promoted to higher energy levels and making the atoms unstable. As the electrons return to lower levels, they're giving off their energy as photons of red light—and that's why the bar seems to glow red. The fire gives off light for exactly the same reason.

Old-style electric lamps work this way too. They make light by passing electricity through a very thin wire filament so it gets incredibly hot. Excited atoms inside the hot filament turn the electrical energy passing through them into light you can see by constantly giving off photons. When we make light by heating things, that's called incandescence. So old-style lamps are sometimes called incandescent lamps.

You can also get atoms excited in other ways. Energy-saving light bulbs that use fluorescence are more energy efficient because they make atoms crash about and collide, making lots of light without making heat. In effect, they make cold light rather than the hot light produced by older-style, energy-wasting bulbs. Creatures like fireflies make their light through a chemical process using a substance called luciferin. The broad name for the various different ways of making light by exciting the atoms inside things is luminescence.

(Let's note in passing that light has some other interesting effects when it gets involved in chemistry. That's how photochromic sunglass lenses work.)

Photo of a rainbow.

Light of many colors

Color (spelled "colour" in the UK) is one of the strangest things about light. Here's one obvious riddle: if we see things because sunlight is reflected off them, how come everything isn't the same color? Why isn't everything the color of sunlight? You probably know the answer to this already. Sunlight isn't light of just one color—it's what we call white light, made up of all the different colors mixed together. We know this because we can see rainbows, those colorful curves that appear in the sky when droplets of water split sunlight into its component colors by refracting (bending) different colors of light by different amounts.

Photo: A rainbow splits sunlight ("white" light) into its component colors because it bends different colors (wavelengths of light) by different amounts. Shorter wavelengths are bent more than longer wavelengths, so blue light is bent more than red. That's why blue is always on the inside of a rainbow and red is on the outside.

Why does a tomato look red? When sunlight shines on a tomato, the red part of the sunlight is reflected back again off the tomato's skin, while all the other colors of lights are absorbed (soaked into) the tomato, so you don't see them. That's just as true of a blue book, which reflects only the blue part of sunlight but absorbs light of other colors.

Artwork showing how a red tomato reflects only red light and absorbs light of other colors

Why does a tomato appear red and not blue or green? Think back to how atoms make light. When sunlight falls on a tomato, the incoming light energy excites atoms in the tomato's skin. Electrons are promoted to higher energy levels to capture the energy, but soon fall back down again. As they do so, they give off photons of new light—and that just happens to correspond to the kind of light that our eyes see as red. Tomatoes, in other words, are like precise optical machines programmed to produce photons of red light when sunlight falls on them.

If you shone light of other colors on tomatoes, what would happen? Let's suppose you made some green light by passing sunlight through a piece of green plastic (something we call a filter). If you shone this on a red tomato, the tomato would appear black. That's because tomatoes absorb green light. There is simply no red light for them to reflect.

Photo: A tomato reflects the red part of sunlight and absorbs all the other colors.

It's not how it is—it's how you see it

Many of the things we think are true of the world turn out to be true only of ourselves. We think tomatoes are red, but in fact we only see them that way. If our eyes were built differently, we might see the light photons that tomatoes produce as light of a totally different color. And there's no real way any of us can be sure that what we see as "red" is the same as what anyone else sees as red: there's no way to prove that my red is the same as yours. Some of the most interesting aspects of the things we see come down to the psychology of perception (how our eyes see the world and how our brains make sense of that), not the physics of light. Color blindness and optical illusions are two examples of this.

Understanding light is a brilliant example of what being a scientist is all about. Science isn't like other subjects. It's not like history (a collection of facts about past events) or law (the rights and wrongs of how people behave). It's an entirely different way of thinking about the world and making sense of it. When you understand the science of light, you feel you've turned part of the world inside out—you're looking from the inside, seeing everything in a totally new way, and understanding for the first time why it all makes sense. Science can throw a completely different light on the world—it can even throw light on light itself!

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Text copyright © Chris Woodford 2008. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2008) Light. Retrieved from http://www.explainthatstuff.com/light.html. [Accessed (Insert date here)]

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