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: Ordinary light looks white, but if you shine it through a
prism (wedge) of glass, you can see that it's really made from a whole spectrum of colors.
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
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!
Most natural light floods into our world from the Sun, shown here in a dramatic closeup,
emitting a blast of radiation called a solar flare.
Photo courtesy of NASA Solar Dynamics Observatory (SDO) .
Is light a particle or a wave?
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 by William Thomas Fry 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.
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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
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!
In fact, it's six segments of the huge mirror from the James Webb Space Telescope.
Picture by courtesy of NASA.
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.
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
Photo: Refraction makes a drinking straw look bent (the top and bottom appear not to be joined up)
when it's standing in a jug of water..
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 up above.
As the beams cross the junction, they bend quite noticeably.
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, 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 eyelids and eyelashes. The tighter
you close your eyes, the more the light spreads (until it disappears
when you close your eyes completely).
Artwork: When light from a laser (1) passes through a narrow slit (2), the waves spread out (3) and form a diffraction pattern of light and dark bands (4). Different numbers, shapes, and sizes of slits produce more complex diffraction patterns.
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.
Photo: Thin-film interference makes the colors you see swirling around on the surface of soap bubbles.
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.
Interference is very colorful, but it has practical uses too. A technique called
interferometry can use interfering laser beams to measure incredibly
small distances.
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Where does light come from?
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.
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.
How atoms make 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:
They start off in their stable "ground state" with electrons in their normal places.
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."
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.
Photo: A glow stick makes "cold light" using luminescence. Photo by
Demetrius Kennon courtesy of US Navy.
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.)
Light of many colors
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.
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.
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.
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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Don't want to read our articles? Try listening instead
Optics for kids: Simple and fun introductory site from the Optical Society of America.
Books
For younger readers
The Illuminating World of Light with Max Axiom, Super Scientist by Emily Sohn and Nick Derington. Capstone, 2019. A 32-page, app-linked graphic novel (comic-style) introduction for ages 8–14, aimed at engaging reluctant readers who might not pick up an ordinary school science book.
Light in a Flash by Georgia Amson-Bradshaw. Rosen, 2019/Franklin Watts, 2017. Facts, quizzes, and experiments lighten this 32-page introduction for ages 7–9.
Scientific Pathways: Light by Chris Woodford. Rosen, 2013. This is one of my own books, also for ages 9–12, and it briefly charts the history of our efforts to understand light (Previously published as Routes of Science: Light, Blackbirch, 2004.)
Light by David Burnie. DK, 1998. One of the well-known DK Eyewitness books combining science, technology, and history in an easily digestible volume. Best for ages 9–12 (though interesting for older people too).
Light: A Very Short Introduction by Ian Walmsley. Oxford, 2015. A solid introduction that takes us (in order) through light rays, waves, duality, relativity, and quantum theory. Quite a lot is compressed into just over 100 pages so the going (for beginners) isn't always easy.
QED: The Strange Theory of Light and Matter by Richard P. Feynman. Penguin, 2007 (reprinted in numerous editions). One of the 20th-century's greatest physicists explains the interactions between light and electrons.
Textbooks
Optics by Eugene Hecht. Addison-Wesley, 2016. The classic undergraduate textbook on light and optics—and the one I used myself some years ago.
Optics by K.K.Sharma. Academic Press, 2006. An alternative textbook for students, but with more about optical applications.
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