We see things every day, from the moment we get up in the
morning until we go to sleep at night. We look at everything
around us using light. We appreciate kids' crayon drawings,
fine oil paintings, swirling computer graphics, gorgeous
sunsets, a blue sky, shooting stars and rainbows. We rely on
mirrors to make ourselves presentable, and sparkling gemstones
to show affection. But did you ever stop to think that when we
see any of these things, we are not directly connected to it?
We are, in fact, seeing light -- light that somehow left
objects far or near and reached our eyes. Light is all our
eyes can really see.
In this edition of HowStuffWorks,
we will look at light from many different angles to show you
exactly how it works!
Ways of Thinking About Light You have
probably heard two different ways of talking about light:
There is the "particle" theory, expressed in part by the
There is the "wave" theory, expressed by the term
From the time of the ancient
Greeks, people have thought of light as a stream of tiny
particles. After all, light travels in straight lines
and bounces off a mirror much like a ball bouncing off a wall.
No one had actually seen particles of light, but even now it's
easy to explain why that might be. The particles could be too
small, or moving too fast, to be seen, or perhaps our eyes see
right through them.
The idea of the light wave came from Christian
Huygens, who proposed in the late 1600s that light acted
like a wave instead of a stream of particles. In 1807,
Thomas Young backed up Huygens' theory by showing that
when light passes through a very narrow opening, it can spread
out, and interfere with light passing through another opening.
Young shined a light through a very narrow slit. What he saw
was a bright bar of light that corresponded to the slit. But
that was not all he saw. Young also perceived additional
light, not as bright, in the areas around the bar. If light
were a stream of particles, this additional light would not
have been there. This experiment suggested that light spread
out like a wave. In fact, a beam of light radiates
outward at all times.
Albert Einstein advanced the theory of light further
in 1905. Einstein considered the photoelectric effect,
in which ultraviolet light hits a surface and causes electrons
to be emitted from the surface. Einstein's explanation for
this was that light was made up of a stream of energy packets
Modern physicists believe that light can behave as both a
particle and a wave, but they also recognize that either view
is a simple explanation for something more complex. In this
article, we will talk about light as waves, because this
provides the best explanation for most of the phenomena our
eyes can see.
What is Light? Why is it that a beam of
light radiates outward, as Young proved? What is really going
on? To understand light waves, it helps to start by discussing
a more familiar kind of wave -- the one we see in the water.
One key point to keep in mind about the water wave is that it
is not made up of water: The wave is made up of energy
traveling through the water. If a wave moves across a pool
from left to right, this does not mean that the water on the
left side of the pool is moving to the right side of the pool.
The water has actually stayed about where it was. It is the
wave that has moved. When you move your hand through a filled
bathtub, you make a wave, because you are putting your energy
into the water. The energy travels through the water in the
form of the wave.
All waves are traveling energy, and they are usually moving
through some medium, such as water. You can see a diagram of a
water wave in Figure 1. A water wave consists of water
molecules that vibrate up and down at right angles to the
direction of motion of the wave. This type of wave is called a
Light waves are a little more complicated, and they
do not need a medium to travel through. They can travel
through a vacuum. A light wave consists of energy in the form
of electric and magnetic fields. The fields vibrate at right
angles to the direction of movement of the wave, and at right
angles to each other. Because light has both electric and
magnetic fields, it is also referred to as electromagnetic
Light waves come in many sizes. The size of a wave is
measured as its wavelength, which is the distance
between any two corresponding points on successive waves,
usually peak-to-peak or trough-to-trough (Figure 1). The
wavelengths of the light we can see range from 400 to
700 billionths of a meter. But the full range of wavelengths
included in the definition of electromagnetic radiation
extends from one billionth of a meter, as in gamma rays, to
centimeters and meters, as in radio
waves. Light is one small part of the spectrum.
Light waves also come in many frequencies. The
frequency is the number of waves that pass a point in
space during any time interval, usually one second. It is
measured in units of cycles (waves) per second, or Hertz (Hz).
The frequency of visible light is referred to as
color, and ranges from 430 trillion Hz, seen as red, to
750 trillion Hz, seen as violet. Again, the full range of
frequencies extends beyond the visible spectrum, from less
than one billion Hz, as in radio waves, to greater than 3
billion billion Hz, as in gamma rays.
As noted above, light waves are waves of energy. The amount
of energy in a light wave is proportionally related to its
frequency: High frequency light has high energy; low frequency
light has low energy. Thus gamma rays have the most energy,
and radio waves have the least. Of visible light,
violet has the most energy and red the least.
Light not only vibrates at different frequencies, it also
travels at different speeds. Light waves move through a vacuum
at their maximum speed, 300,000 kilometers per second or
186,000 miles per second, which makes light the fastest
phenomenon in the universe. Light waves slow down when they
travel inside substances, such as air, water, glass or a diamond.
The way different substances affect the speed at which light
travels is key to understanding the bending of light, or
refraction, which we will discuss later.
So light waves come in a continuous variety of sizes,
frequencies and energies. We refer to this continuum as the
electromagnetic spectrum (Figure 2). Figure 2 is
not drawn to scale, in that visible light occupies only
one-thousandth of a percent of the spectrum.
Producing a Photon Any light that you see is
made up of a collection of one or more photons propagating
through space as electromagnetic waves. In total darkness, our
eyes are actually able to sense single photons, but generally
what we see in our daily lives comes to us in the form of
zillions of photons produced by light sources and reflected
off objects. If you look around you right now, there is
probably a light source in the room producing photons, and
objects in the room that reflect those photons. Your eyes
absorb some of the photons flowing through the room,
and that is how you see.
There are many different ways to produce photons, but all
of them use the same mechanism inside an atom to do
it. This mechanism involves the energizing of electrons
orbiting each atom's nucleus. How Nuclear
Radiation Works describes protons, neutrons and electrons
in some detail. For example, hydrogen atoms have one electron
orbiting the nucleus. Helium atoms have two electrons orbiting
the nucleus. Aluminum atoms have 13 electrons orbiting the
nucleus. Each atom has a preferred number of electrons
orbiting its nucleus.
Electrons circle the nucleus in fixed orbits -- a
simplified way to think about it is to imagine how satellites
orbit the Earth. There's a huge amount of theory around
electron orbitals, but to understand light there is just one
key fact to understand: An electron has a natural orbit that
it occupies, but if you energize an atom you can move
its electrons to higher orbitals. A photon of light is
produced whenever an electron in a higher-than-normal orbit
falls back to its normal orbit. During the fall from
high-energy to normal-energy, the electron emits a photon -- a
packet of energy -- with very specific characteristics. The
photon has a frequency, or color, that exactly matches the
distance the electron falls.
There are cases where you can see this phenomenon quite
clearly. For example, in lots of factories and parking lots
you see sodium vapor lights. You can tell a sodium
vapor light because it is very yellow when you look at it. A
sodium vapor light energizes sodium atoms to generate photons.
A sodium atom has 11 electrons, and because of the way they
are stacked in orbitals one of those electrons is most likely
to accept and emit energy (this electron is called the 3s
electron, and is explained on this
page). The energy packets that this electron is most
likely to emit fall right around a wavelength of 590
nanometers. This wavelength corresponds to yellow light. If
you run sodium light through a prism, you do not see a rainbow
-- you see a pair of yellow lines.
Probably the most common way to energize atoms is with
heat, and this is the basis of incandescence. If you
heat up a horseshoe with a blowtorch, it will eventually get
red hot, and if you heat it enough it gets white hot. Red is
the lowest-energy visible light, so in a red-hot object the
atoms are just getting enough energy to begin emitting light
that we can see. Once you apply enough heat to cause white
light, you are energizing so many different electrons in so
many different ways that all of the colors are being generated
-- they all mix together to look white, as explained in one of
the sections below.
Heat is the most common way we see light being generated --
a normal 75-watt incandescent bulb is generating light by
using electricity to create heat. However, there are lots of
other ways to generate light, some of which are listed below:
Halogen lamps - Halogen
lamps use electricity to generate heat, but benefit from
a technique that lets the filament run hotter.
Gas lanterns - A gas
lantern uses a fuel like natural gas or kerosene as the
source of heat.
Fluorescent lights - Fluorescent
lights use electricity to directly energize atoms rather
than requiring heat.
Lasers - Lasers use
energy to "pump" a lasing medium, and all of the energized
atoms are made to dump their energy at the exact same
wavelength and phase.
Glow-in-the-dark toys - In a glow-in-the-dark
toy, the electrons are energized but fall back to
lower-energy orbitals over a long period of time, so the toy
can glow for half an hour.
Chemical light sticks - A chemical light stick
and, for that matter, fireflies,
use a chemical reaction to energize atoms.
to note from this list is that anything that produces
light does it by energizing atoms in some way.
Making Colors Visible light is light that
can be perceived by the human eye. When you look at the
visible light of the sun, it appears to be colorless,
which we call white. And although we can see this
light, white is not considered to be part of the visible
spectrum (Figure 2). This is because white light is not the
light of a single color, or frequency. Instead, it is made up
of many color frequencies. When sunlight passes through a
glass of water to land on a wall, we see a rainbow on the
wall. This would not happen unless white light were a mixture
of all of the colors of the visible spectrum. Isaac
Newton was the first person to demonstrate this. Newton
passed sunlight through a glass prism to separate the colors
into a rainbow spectrum. He then passed sunlight through a
second glass prism and combined the two rainbows. The
combination produced white light. This proved conclusively
that white light is a mixture of colors, or a mixture of light
of different frequencies. The combination of every color in
the visible spectrum produces a light that is colorless, or
Colors by Addition - You can do a similar
experiment with three flashlights and three different colors
of cellophane -- red, green and blue (commonly referred to
as RGB). Cover one flashlight with one to two layers of red
cellophane and fasten the cellophane with a rubber band (do
not use too many layers or you will block the light from the
flashlight). Cover another flashlight with blue cellophane
and a third flashlight with green cellophane. Go into a
darkened room, turn the flashlights on and shine them
against a wall so that the beams overlap, as shown in
Figure 3. Where red and blue light overlap, you will
see magenta. Where red and green light overlap, you will see
yellow. Where green and blue light overlap, you will see
cyan. You will notice that white light can be made by
various combinations, such as yellow with blue, magenta with
green, cyan with red, and by mixing all of the colors
By adding various combinations of red, green and blue
light, you can make all the colors of the visible spectrum.
This is how computer
monitors (RGB monitors) produce colors.
Colors by Subtraction - Another way to make
colors is to absorb some of the frequencies of light, and
thus remove them from the white light combination. The
absorbed colors are the ones you will not see -- you see
only the colors that come bouncing back to your eye. This is
what happens with paints and dyes. The paint or dye
molecules absorb specific frequencies and bounce back, or
reflect, other frequencies to your eye. The reflected
frequency (or frequencies) are what you see as the color of
the object. For example, the leaves of green plants contain
a pigment called chlorophyll, which absorbs the blue
and red colors of the spectrum and reflects the green.
Here is an absorption experiment that you can try at home:
Take a banana and the blue cellophane-covered flashlight you
made earlier. Go into a dark room, and shine the blue light on
the banana. What color do you think it should be? What color
is it? If you shine blue light on a yellow banana, the yellow
should absorb the blue frequency; and, because the room is
dark, there is no yellow light reflected back to your eye.
Therefore, the banana appears black.
So, if you had three paints or pigments in magenta, cyan
and yellow, and you drew three overlapping circles with those
colors, as shown in Figure 4, you would see that where
you have combined magenta with yellow, the result is red.
Mixing cyan with yellow produces green, and mixing cyan with
magenta creates blue. Black is the special case in which all
of the colors are absorbed. You can make black by combining
yellow with blue, cyan with red or magenta with green. These
particular combinations ensure that no frequencies of visible
light can bounce back to your eyes.
But the color scheme demonstrated in Figure 4 appears to go
against what your art teacher told you about mixing colors,
right? If you mix yellow and blue crayons, you get green, not
black. This is because artificial pigments, such as
crayons, are not perfect absorbers -- they do not absorb all
colors except one. A "yellow" crayon can absorb blue and
violet while reflecting red, orange and green. A "blue" crayon
can absorb red, orange and yellow while reflecting blue,
violet and green. So when you combine the two crayons, all of
the colors are absorbed except for green. Therefore, you see
the mixture as green, instead of the black demonstrated in
So there are two basic ways by which we can see colors.
Either an object can directly emit light waves in the
frequency of the observed color, or an object can
absorb all other frequencies, reflecting back to your
eye only the light wave, or combination of light waves, that
appears as the observed color. For example, to see a yellow
object, either the object is directly emitting light waves in
the yellow frequency, or it is absorbing the blue part of the
spectrum and reflecting the red and green parts back to your
eye, which perceives the combined frequencies as yellow.
When Light Hits an Object When a light wave
hits an object, what happens to it depends on the energy of
the light wave, the natural frequency at which electrons
vibrate in the material and the strength with which the atoms
in the material hold on to their electrons. Based on these
three factors, four different things can happen when light
hits an object:
The waves can be reflected or scattered
off the object.
The waves can be absorbed by the object.
The waves can be refracted through the object.
The waves can pass through the object with no
And more than one of these possibilities can
happen at once. The following five illustrations show these
possibilities, with reflection and scattering illustrated
Transmission - If the frequency or energy of the
incoming light wave is much higher or much lower than the
frequency needed to make the electrons in the material
vibrate, then the electrons will not capture the energy of
the light, and the wave will pass through the material
unchanged. As a result, the material will be transparent to
that frequency of light.
Most materials are transparent to some frequencies, but not
to others. For example, high frequency light, such as gamma
rays and X-rays, will pass through ordinary glass, but lower
frequency ultraviolet and infrared light will not.
You can read more about what makes glass transparent on this
Absorption - In absorption, the frequency of the
incoming light wave is at or near the vibration frequency of
the electrons in the material. The electrons take in the
energy of the light wave and start to vibrate. What happens
next depends upon how tightly the atoms hold on to their
electrons. Absorption occurs when the electrons are held
tightly, and they pass the vibrations along to the nuclei of
the atoms. This makes the atoms speed up, collide with other
atoms in the material, and then give up as heat the energy
they acquired from the vibrations.
The absorption of light makes an object dark or opaque to
the frequency of the incoming wave. Wood is opaque to visible
light. Some materials are opaque to some frequencies of light,
but transparent to others. Glass is opaque to ultraviolet
light, but transparent to visible light.
Reflection and Scattering: The atoms in some
materials hold on to their electrons loosely. In other
words, the materials contain many free electrons that can
jump readily from one atom to another within the material.
When the electrons in this type of material absorb energy
from an incoming light wave, they do not pass that energy on
to other atoms. The energized electrons merely vibrate and
then send the energy back out of the object as a light wave
with the same frequency as the incoming wave. The overall
effect is that the light wave does not penetrate deeply into
In most metals, electrons are held loosely, and are free to
move around, so these metals reflect visible light and appear
to be shiny. The electrons in glass have some freedom, though
not as much as in metals. To a lesser degree, glass reflects
light and appears to be shiny, as well.
A reflected wave always comes off the surface of a
material at an angle equal to the angle at which the incoming
wave hit the surface. In physics, this is called the Law of
Reflectance. You have probably heard the Law of
Reflectance stated as "the angle of incidence equals the angle
You can see for yourself that reflected light has the same
frequency as the incoming wave. Just look at yourself in a
mirror. The colors you see in the mirror's image are the same
as those you see when you look down at yourself. The colors of
your shirt and hair are the same as reflected in the mirror as
they are on you. If this were not true, we would have to rely
entirely on other people to tell us what we look like!
Scattering is merely reflection off a rough surface.
Incoming light waves get reflected at all sorts of angles,
because the surface is uneven. The surface of paper is a good
example. You can see just how rough it is if you look at it
under a microscope. When light hits paper, the waves are
reflected in all directions. This is what makes paper so
incredibly useful -- you can read the words on a printed page
regardless of the angle at which your eyes view the surface.
Another interesting rough surface is Earth's atmosphere.
You probably don't think of the atmosphere as a surface, but
it nonetheless is "rough" to incoming white light. The
atmosphere contains molecules of many different sizes,
including nitrogen, oxygen, water vapor and various
pollutants. This assortment scatters the higher energy light
waves, the ones we see as blue light. This is why the sky
Refraction - Refraction occurs when the energy of
an incoming light wave matches the natural vibration
frequency of the electrons in a material. The light wave
penetrates deeply into the material, and causes small
vibrations in the electrons. The electrons pass these
vibrations on to the atoms in the material, and they send
out light waves of the same frequency as the incoming wave.
But this all takes time. The part of the wave inside the
material slows down, while the part of the wave outside the
object maintains its original frequency. This has the effect
of bending the portion of the wave inside the object toward
what is called the normal line, an imaginary straight
line that runs perpendicular to the surface of the object.
The deviation from the normal line of the light inside the
object will be less than the deviation of the light before
it entered the object.
The amount of bending, or angle of refraction, of
the light wave depends on how much the material slows down the
light. Diamonds would not be so glittery if they did not slow
down incoming light much more than, say, water does. Diamonds
have a higher index of refraction than water, which is
to say that they slow down light to a greater degree.
One interesting note about refraction is that light of
different frequencies, or energies, will bend at
slightly different angles. Let's compare violet light and red
light when they enter a glass prism. Because violet light has
more energy, it takes longer to interact with the glass. As
such, it is slowed down to a greater extent than a wave of red
light, and will be bent to a greater degree. This accounts for
the order of the colors that we see in a rainbow. It is also
what gives a diamond the rainbow fringes that make it so
pleasing to the eye.
Rainbows in Soap Bubbles Have you ever
wondered why soap bubbles are rainbow colored, or why an oil
spill on a wet road has rainbow colors in it? This is what
happens when light waves pass through an object with two
reflective surfaces. When two incoming light waves of the same
frequency strike a thin film of soap, as seen in Figure
5 below, parts of the light waves are reflected from the
top surface, while other parts of the light pass through the
film and are reflected from the bottom surface. Because the
parts of the waves that penetrate the film interact with the
film longer, they get knocked out of sync with the parts of
the waves reflected by the top surface. Physicists refer to
this state as being out of phase. When the two sets of
waves strike the photoreceptors in your eyes, they interfere
with each other; interference occurs when waves add together
or subtract from each other and so form a new wave of a
different frequency, or color.
Basically, when white light, which is a mixture of
different colors, shines on a film with two reflective
surfaces, the various reflected waves interfere with each
other to form rainbow fringes. The fringes change colors when
you change the angle at which you look at the film, because
you are changing the path by which the light must travel to
reach your eye. If you decrease the angle at which you look at
the film, you increase the amount of film the light must
travel through for you to see it. This causes greater interference.
Everything we see is a product of, and is affected by, the
nature of light. Light is a form of energy that travels in
waves. Our eyes are attuned only to those wave frequencies
that we call visible light. Intricacies in the wave nature of
light explain the origin of color, how light travels, and what
happens to light when it encounters different kinds of
Hewitt, Paul G., (1999) Conceptual
Physics, Third Edition, Scott-Foresman-Addison-Wesley,
Inc., Menlo Park, Calif.
Serway, Raymond A, and Jerry S. Faughn, (1999) Holt
Physics, Holt, Rinehart, and Winston, Austin, Texas