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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.
The other way that
we encounter light is in devices that produce light --
incandescent bulbs,
fluorescent bulbs,
lasers,
lightning bugs,
and the sun. Each one uses a different technique to generate
photons.
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 word photon.
There is the
"wave" theory, expressed by the term light wave.
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 called
photons.
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.
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 radiation.
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. 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 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.
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Figure 2
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So light waves
come in a continuous variety of sizes, frequencies and energies. We
refer to this continuum as the electromagnetic spectrum. 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.
Indiglo watches
- In
Indiglo watches, voltage energizes phosphor atoms.
Chemical light
sticks - A chemical light
stick and, for that matter,
fireflies, use a chemical reaction to energize atoms.
The thing 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. 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 was 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 white.
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. 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
together.
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 - red, green, blue monitors) produce colors.
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:
S T
A R
When treating an object some of the
lasers waves will Scatter,
Transmit,
Absorb and some will
Reflect. Hence the acronym
STAR.
The waves can be
reflected or scattered off the object. The object can
absorb the waves. The waves can be refracted through the object.
The waves can pass through the object with no effect. And more
than one of these possibilities can happen at once. The following five
illustrations show these possibilities, with reflection and scattering
illustrated separately.
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 looks blue.
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 page.
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:
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 the material.
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.
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!
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.
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Figure 5 |
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 materials.
References
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
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