4 - 2 Light and Color
LIGHT AND COLOR
Name(s) ____________________________________
Do you know why things are the color that they are? Have you ever wondered why the
ocean is blue? Do you know why grass is green? Have you ever wondered what makes the sun
and the sky turn red as the sun goes down?
Most people think that objects look a certain color simply because they are that color. For
example, we say, "Roses are red and violets are blue," not "Roses just look red, and violets just
look blue." We think of objects as having color, not that they just appear to have color. We
want to believe that a red rose would still be red even if we were to place it somewhere that was
so dark that we could not even see the rose. It might be too dark to see, but the rose would
certainly still be red, wouldn't it?
The problem with this common sense "model" for colors is that it cannot be used to explain
a variety of color phenomena we experience everyday. It cannot be used to explain why the sky
is blue during the day, red at dusk, and black at night. It does not explain why the "blue" shirt
we bought at the store looks purple when we get it home. It does not account for how television
or computer screens can display objects that appear to be yellow even though television and
computer screens cannot make the color yellow. And it certainly cannot begin to explain the
formation of a rainbow.
The activities in this section will introduce a scientific model for color that is an extension
of the photon model for light. At first, it may seem awkward to try and explain color by
referring to photons, but over time the photon model should become an indispensable way for
you to think about color.
The Photon Model
The diagram on the next page is meant to portray the photon model of light. The person
shown in the diagram is able to see the box on the right because photons are coming out of the
lamp, bouncing of the box, and then going into the person's eyes. This picture represents a
somewhat simplified version of the photon model, because the photons bouncing off the box
should really be shown scattering in every direction, not just towards the person. However, it
would be very confusing to show all of the photons, so just the ones that are bouncing towards
the person are shown.
Notice that more photons are shown hitting the box than are shown bouncing off. This is
because some photons are absorbed when they hit the box. A perfectly black box would
absorb all the photons that hit it. A perfectly white box would reflect all the photons.
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4 - 2 Light and Color
A Diagram of the Photon Model
Note: Some photons are absorbed when they hit the box.
It is important to remember that, according to the photon model, the man in the picture does
not actually "see" the box. Rather, what he sees are the photons coming into his eyes that have
bounced of the box. Therefore, the man's brain determines what is true about the way the box
looks, including the color of the box, based upon the photons that are going into his eyes.
The photons themselves must have a way of telling the man about the color of the box.
Photons Come in Many Colors
One way that photons could tell us about the color of an object would be if the photons
themselves came in different colors. As a matter of fact, we can demonstrate that photons do
indeed come in many colors by looking at the world through special glasses that have
diffraction grating lenses.
Activity #1 Separating Colors with Diffraction Glasses
The room lights should be dim or off for the following activities.
Obtain a pair of diffraction grating glasses for each member of your group, plus one laser
pointer to be shared. There should also be a few regular light sources positioned around the
room that you can see from your seat. Be careful never to look at a bright light source
(especially the sun or a laser beam!) or try to drive a car while you are wearing the
diffraction grating glasses.
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1. Without using your diffraction grating glasses, look at one of the regular light sources in
the room. Note that the light coming from the bulb appears to be white.
2. Now, put on your diffraction grating glasses, and notice that the main beam of light still
looks white, but there are also very colorful light patterns all around the main beam.
Somehow, diffraction grating glasses are able to produce all the colors of the rainbow from
a source of "white" light.
3. Take off your diffraction glasses, and this time shine the laser at a piece of paper in front of
you. Remember; never look directly into the laser beam or directly at its reflection
from a mirror! It is only safe to look at the light from the laser after it has reflected off an
object such as the wall or a piece of paper.
What color is the laser light?
What colors do you think you will see if you were to look at the laser light through your
diffraction grating glasses? (Remember, only look at the laser light after it has reflected
from the paper.) Will you see the same colorful patterns of light as you saw when you
looked at the regular light source, or will there be a difference? Write your prediction
below.
4. Shine the laser at the the paper, and look at its reflection through your diffraction grating
glasses. What do you see? Was you prediction correct? Describe below the colors you
can see through your glasses.
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Apparently diffraction grating glasses are able to separate white light into all the colors of
the rainbow, but they do not turn single colors of the rainbow (in this case red) into other
colors. How do diffraction glasses manage to do this? The explanation depends upon us
imagining that photons come in different colors.
A Photon Model for Color
All the colors that we see around us can be explained if we imagine that photons come in
different colors. For simplicity, we will imagine that photons can only be one of the seven
colors of the rainbow: red, orange, yellow, green, cyan, blue, and violet. The colors that we
perceive depend upon the colors of the photons that are going into our eyes. When we see the
red light of a laser, then only red photons are going into our eyes. If we see the reflection of a
green laser light, then that would mean that only green photons are entering our eyes.
If two or more differently colored photons are entering your eye at the same time, then your
brain interprets the color you see to be a kind of average of those photon colors. For example,
if equal numbers of red and yellow photons go into your eye, then your brain interprets the
color to be orange, even though orange photons are not actually going into your eye. A
mixture of red and blue photons will appear to be a purple color. When photons of all the
colors of the rainbow go into your eye at the same time, your brain interprets that color
to be white!
The diagram below depicts a man looking at a lamp that produces normal "white" light.
The photons traveling towards the man have been labeled as R, O, Y , G, C , B, and V to
designate photons that are red, orange, yellow, green, cyan, blue, and violet. (A normal light
bulb would produce photons of all these colors.)
Even though many differently colored photons are entering the man's eyes, his brain can
only interpret a single color at a time. Since a combination of photons of all colors of the
rainbow average out to be white, the man thinks that the light is white. Keep in mind that there
are no white photons. We only think we see white when we see all the colors at once.
R
O
Y
B
R
C
V
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4 - 2 Light and Color
The diagram below shows a man looking at a beam of light from a lamp after it reflects off
a white piece of paper. Since white paper is good at reflecting photons, all the different colors
of photons are still reaching his eyes, and the spot of light on the paper looks white to him.
R
Y
O
V
B
R
C
Y
V
G
G
B
O
R
C
B
O
G
C
V
B
Y
R
G
O
C
V
B
R
Y
The man below is looking at a spot on a white piece of paper from a red laser beam. Since
the laser only produces red photons, and since the "white" paper reflects all photons well
(including red ones), the man sees a red spot on the paper.
LASER
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
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Your diffraction grating glasses have the special ability to separate photons according to
their color. The diagram below shows what happens when "white" light passes through a
diffraction grating. (In order to keep the diagram from being too cluttered, only blue, green
and red photons have been shown.) Notice that some of the photons go straight through the
diffraction grating glasses. The rest bend off to the side, with the color of the photon
determining the amount that the photon changes direction. Once the photons have changed
direction according to their color, we can see individual beams of differently colored photons.
R
R
R
G
R
G
R
G
G
B
G
B
R
R
G
B
B
R
B
B
B
G
R
B
B
G
R
B
G
R
R
G
G
B
B
R
R
G
G
B
Diffraction Grating Glasses
The diagram below shows red laser light going through diffraction grating glasses. Since the
laser only gives off red photons, only red light can be seen on the other side of the diffraction
grating glasses.
R
R
R
R
R
R
R
LASER
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
Diffraction Grating Glasses
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R
4 - 2 Light and Color
Our eyes alone cannot tell us what colors of photons we are seeing when the photons are all
mixed together, because our brains interpret the mix of colors as being a single color.
However, the diffraction grating glasses can tell us what colors are present, because they
separate out the photons according to color so that we can see them. Therefore, diffraction
grating glasses are useful tools in discovering what colors of photons are present in light.
Activity #2 Seeing True Colors with Diffraction Glasses
It is best if the room lights are very dim for these activities. If you cannot dim the lights in
this room, then try moving to another, darker location to do this activity.
Obtain three differently colored LED penlights (red, blue and green). Have your
diffraction grating glasses ready, but do not put them on until you are instructed to do so.
Warning: Do not point the lights directly towards your eyes. They are quite bright. View the
lights from the side or from slightly behind.
A. Red Penlight
Observation: What color does this light appear to be when you are not wearing your
diffraction grating glasses?
Hypothesis: What color photons do you think the penlight is giving off? Explain why you
think so.
Testing the Hypothesis: Use your diffraction grating glasses to determine what color
photons actually are being given off by the light. Compare your answer to your
hypothesis, and explain any differences between them. (Ask your instructor for help if
you are unsure of your results.)
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B. Blue Penlight
Observation: What color does this light appear to be when you are not wearing your
diffraction grating glasses?
Hypothesis: What color photons do you think the penlight is giving off? Explain why you
think so.
Testing the Hypothesis: Use your diffraction grating glasses to determine what color
photons actually are being given off by the light. Compare your answer to your
hypothesis, and explain any differences between them.
C. Green Penlight
Observation: What color does this light appear to be when you are not wearing your
diffraction grating glasses?
Hypothesis: What color photons do you think the penlight is giving off? Explain why you
think so.
Testing the Hypothesis: Use your diffraction grating glasses to determine what color
photons actually are being given off by the light. Compare your answer to your
hypothesis, and explain any differences between them.
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Activity #3
Testing Color Filters
Obtain a set of plastic color filters and three differently colored LED pen lights (red, blue and
green) for your group.
A filter is defined to be an object that will let some materials or objects through, but not others.
For example, a coffee filter allows coffee and water to pass through, but not coffee grounds. A
water filter allows water through, but not dirt.
Look through each of your filters (one at a time) at a white light source. Why do you think
they call these filters? For example, what does a red color filter do?
The diagram below shows white light from a lamp striking a red filter. Complete the diagram
by showing the photons that would emerge on the right side of the filter. Have your
instructor or the class assistant check your work before you move on.
R
O
Y
B
C
V
G
Red Filter
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The partial diagrams below show light sources shining at filters. Complete the diagrams by
showing the photons moving towards the filters from the left, and then also indicate if any of the
photons would get through. In every case assume that the light sources and the filters are
perfect.
Green light
Red filter
White light
Green filter
Blue light
Blue filter
Green light
Blue filter
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Activity #4 Are Color Filters Perfect?
A perfect red filter would only let red photons through. The photons of other colors would get
absorbed, or else they would reflect off. A perfect green filter would only let green photons
through, and it would stop all other photons from getting through.
Use your diffraction grating glasses and find a way to test and see how good your red, yellow,
green, and blue filters operate. Do they just allow one color of photon through, or do several
different colors get through? Describe below how you did your tests. For each filter, indicate
which colors of photons got through by circling those colors. If only a few photons of a certain
color get through, underline that color.
Describe how you did your tests:
Red Filter Which photons can get through?
Red
orange
yellow
green
cyan
blue
violet
cyan
blue
violet
cyan
blue
violet
cyan
blue
violet
Yellow Filter Which photons can get through?
Red
orange
yellow
green
Green Filter Which photons can get through?
Red
orange
yellow
green
Blue Filter Which photons can get through?
Red
orange
yellow
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Activity #5
Testing Your Conclusions
In Activity # 2 (pp. 27-28) you discovered that the red, green and blue penlights did not give
off just red, green and blue photons. Other colors were given off as well. In Activity #4
(p. 31) you saw that color filters are never perfect either. They often let through colors that
you would not expect.
Experiment by looking at the pen lights through the color filters and see that indeed light often
gets through the filters even though you would not expect it to do so if the lights and the filters
were perfect.
There is one color filter that will almost completely block the light from one of the penlights.
Find this combination of light and filter, and write down what it is.
Light color _____________________
filter color _________________
Is this result consistent with what you found in Activities # 2 and # 4? Explain why you think
so.
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4 - 2 Light and Color
The Color of Objects
Activity #6 The Dancing Bears
The room must be nearly completely dark for these activities. It is best done where the lights
can be turned off and there are no windows to the outside.
Obtain three differently colored LED pen lights (red, blue and green), and six small plastic
bears, each one a different color of the rainbow.
Line up the bears in order such that their color sequence matches that of a rainbow. With
all the lights off, illuminate the bears using the penlights. Try this with individual penlights on,
and with combinations of the lights on.
1. Which bears look darkest when the red light is on alone? Why do you think that is?
2. Which bears look darkest when the blue light is on alone? Why do you think that is?
3. What combination of lights must be on in order to make all the bears at the same time look
the most "natural"?
4. Why do you suppose the orange bear never seems to look its natural color?
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Discussion
The Dancing Bear activity clearly demonstrates that the color of an object depends upon
the color of the light shining upon the object. This ought not to be too surprising, because the
photons themselves must tell us about the color of an object. For example, if an object
normally looks red, then it must be reflecting red photons into your eyes. But, if there are no
red photons coming from the light source, then the object cannot possibly look red, because it
cannot reflect any red photons to you.
The diagram below depicts what occurs when a red box is illuminated by a white light
source. Only the red photons are reflected from the box and into the man's eyes. Of course,
this diagram is idealized, because no surface would just reflect red photons. A few other colors
would get reflected occasionally as well. Most of the reflected photons, however, would be red
if the box did indeed look red.
R
O
Y
B
V
C
V
Y
R
G
B
O
G
R
C
O
R
R
R
R
R
The diagram below shows a perfectly blue light source shining at the "red" box. Since blue
photons will not reflect from a "red" box, the box would appear to be very dark to the man.
Blue light
B
B
B
B
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4 - 2 Light and Color
Each diagram below depicts a light source shining on a box. The color of the light source,
plus the color that the box would look with a white light source, have been indicated. For each
diagram draw the photons that would be present, and state what color the box would appear to
be with the given light source.
Red light
Green
White light
Reddish/
Orange
Blue/Green
Green
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Homework Questions
1. As more and more lights of many different colors are projected onto a white piece of paper,
the bright spot illuminated on the paper looks more and more white. On the other hand, as
more and more paint of different colors gets mixed together and brushed onto a piece of
white paper, the painted spot appears more and more black. Explain why you obtain these
two different results for mixing colors in these two different ways.
2. Given what you know about light absorption and reflection, explain why black-asphalted
roads heat up more in the summer than white gravel roads.
3. What color would a red rose appear to be if it were placed in blue light?
4. Write a report to explain why the sky is blue during the day, black at night, and often orange
or red at sunrise and sunset.
5. a. People living in hot sunny climates often wear white clothing. Use your model of light to
explain why this is so.
b. People who live in the very hottest and sunniest climates of all actually wear black
clothing. In this case their outer clothing becomes much hotter than their skin temperature,
causing breezes to blow through their clothing. Explain how this happens. (Hint: Think
back to Unit 1)
6. What color would a white piece of paper appear under the following lights:
WHITE
RED
GREEN
BLUE
YELLOW
7. Both light and sound can be represented with a wave model. Therefore, it is not surprising
that sound and light have many things in common. List as many common characteristics
between light and sound as you can. (One example would be that both light and sound can
reflect off objects.) Also list a few characteristics that definitely differ between the two.
8. Given that color pigments are not perfect in their ability to absorb light, explain why you
would expect a mixture of red paint and yellow paint to make orange paint.
9. Get a sheet of white paper, a red report cover (which will act as a red filter), and a box of
markers. Use your markers to draw a picture that contains a hidden picture or message that is
revealed when the picture is viewed through the red report cover. (Your instructor can show
you an example of this.) Explain how this works.
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Models of Light
The Photon Model of Light
Content Overview
A good scientific model for light must be useful for explaining most of the phenomena that
we experience with light and with seeing. Scientists have actually developed two different
models for light that have proven to be very useful in this regard; the Photon Model and the
Wave Model. Of these two, the Photon Model is easier to understand and to apply.
The Photon Model for light states that light is a stream of photons; small bundles of energy
that have no mass and that travel in straight lines at high rates of speed. Photons move at a rate
of about 186,000 miles in a second! We have pictured these bundles of energy as tiny balls
flying through the air.
When photons strike an object they will either reflect (bounce off), get absorbed, or get
transmitted (pass through). Glass is an example of a material that transmits photons well. That
is to say, light passes through glass easily. If an object absorbs light, then the energy from the
photons is given to the object, and the object generally heats up. Dark objects tend to absorb
photons better than light objects, and so dark objects heat up faster in light than do light objects.
We "see" an object only when photons reflect off that object and go into our eye.
Photons come in different "colors". The amount of energy a photon has determines its
color. Blue photons have slightly more energy than cyan photons, which have slightly more
energy than green photons, which have slightly more energy than yellow photons, etc. Red
photons are the least energetic photons that we can see. The colors that we perceive with our
eyes are simply a combination of the colors of the photons that are entering our eyes. When
photons of all the visible colors of the rainbow enter our eyes at once, we perceive the color to be
white. Most of the colors seen by the human eye can be replicated by combining the right
proportions of red, green, and blue photons. For example, combinations of just red, green, and
blue lights produce the colors we see on color TVs and computer monitors.
Many photons exist with energies that are higher than violet photons or with energies that
are lower than red photons. Human eyes cannot detect these photons, so this is often called nonvisible light. X-rays, ultraviolet rays, infrared light, microwaves, and radio waves are all
examples of photons that are invisible to the human eye. The visible colors of the rainbow make
up a very small portion of all the photons that actually exist.
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Models of Light
The Wave Model of Light
Content Overview
In addition to the Photon Model of Light, scientists long ago also developed a Wave Model
of Light. Perhaps it is more common to hear people speaking of light waves than it is to hear
them refer to photons, but it is also probably true that the wave model for light is harder to
understand than the photon model. Nevertheless, there are certain phenomena associated with
light that are more naturally or more easily described using a wave model than the photon model,
and so both models are used extensively in science.
WAVES
In order to understand the Wave Model of Light, you must first understand something about
waves in general. You can start by thinking about water waves. There are large waves in oceans
and other bodies of water, and smaller waves in swimming pools, bathtubs, and sinks. Perhaps
most people picture large ocean waves breaking near the shore when they are asked to think
about waves. Unfortunately, these breaking waves, highly coveted by surfers, represent
somewhat of a special case when it comes to waves. For most of this discussion, think about the
waves further out from shore that can travel more or less undisturbed through the water.
Nonetheless, one thing that breaking waves at the shore show well is that all waves carry energy!
What is a wave? It is a traveling "disturbance" that carries energy from one place to
another, but with no overall movement of matter. You can convince yourself that water waves
carry energy without actually carrying away any water if you position yourself in a boat in the
middle of a lake and begin to tap the water with your hand. You will clearly see a succession of
waves moving away from the boat, and these waves will jostle any objects they encounter as
they travel along, but you will not find that the level of water in the part of the lake you are
floating in is decreasing. The waves you produced carried away energy from you, but they did
not actually carry away any water.
Water waves can be described very well by using a particular mathematical model called a
wave equation. As it turns out, this same mathematical wave equation can be used to describe
many other phenomena as well. Whenever energy is being carried from one place to another
without the overall movement of matter, then the wave equation works as a good model for the
phenomena. Scientists classify all such phenomena as waves. Sound and light are two examples
of such phenomena.
The sketch on the next page shows a side view of part of a simple wave (such as a water
wave) traveling left to right at speed v. The distance between successive peaks is called the
wavelength and is denoted by λ. The amplitude A of the wave gives the maximum displacement
from the equilibrium (no-wave) situation. The crests of the wave are a distance A above
equilibrium and the troughs a distance A below the equilibrium situation.
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Models of Light
λ
2Α
v
Imagine a piece of wood floating in a lake or in the middle of the ocean. As waves pass by, the
wood moves up and down with the water (always remaining on the surface of the water), but it
would not travel along with the wave. The total time it takes for the piece of wood to move up
and then back down again is called the period of the wave, and the letter T usually denotes it.
The frequency of the wave is the number of up and down oscillations that occur per unit time,
and it is given by frequency f = 1/T. For example, if the wood bobs up and down so that it
returns back to the same location every 2 seconds, then the period of the wave is 2 seconds, and
the frequency is 1/2 cycles per second.
Think about what the wave would look like a time T after the time shown in the diagram.
An object oscillating in the wave must return to the same location--so the wave will look just like
the one in the diagram, except it will have moved over exactly a distance λ. That way an object
that was at a peak will again be at a peak. Since distance = speed x time, we conclude that λ = v
T. And since T = 1/f, we have λ = v/f or λf = v. This last equation gives an important relation
between wavelength, frequency, and speed of the wave.
Example:
If a wave has frequency 2 cycles per second, and the distance between crests is 1.5 meters, what
is the speed of the wave?
Answer: The speed is given by v = λf where λ = 1.5 m and f = 2 cycles per second,
v = 1.5 x 2 m/s = 3 m/s.
At a sophisticated level, we can think of light as oscillating electric and magnetic fields
carrying energy. Because of the oscillations of these fields we can talk about light as a wave,
and we can ascribe particular wavelengths to all the colors of the rainbow. Different
wavelengths refract and diffract differently, which allows us to separate colors using prisms or
diffraction gratings. And, like other waves, light shows interference effects. That is, when light
waves are combined they can cancel each other out in some cases or reinforce each other (add
together to form a bigger wave) in other cases. Diffraction gratings work on the basis of
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Models of Light
interference. The bright colors that we see arise at regions where light waves from different slits
in the grating reinforce each other. Different wavelengths reinforce in different places, and so
each wavelength becomes visible at different places.
Visible light is actually only a small part of what is called “electromagnetic radiation.”
Beyond red (longer wavelengths) are the infrared, microwaves (such as are used in microwave
ovens), and radio waves. Beyond violet (shorter wavelengths) are ultraviolet light and x-rays.
All are electromagnetic radiation. Visible light is just a narrow band in which the sun is most
intense and to which our eyes are most sensitive. The visible colors of light, along with their
approximate wavelengths (in nanometers) are given below.
RED
620-700
ORANGE
600
YELLOW
580
GREEN
CYAN
500-550
490
BLUE
VIOLET
470
400-450
There are many characteristics of light that could be studied using the wave model. One
of these is polarization, the direction of oscillation of the electric field, which is essential to
understanding how Polaroid glasses work.
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