IDS 102
The Nature of Light
Major concepts we will cover in this module are: electromagnetic radiation, intensity,
waves, wavelength, reflection, transmission, and absorption.
WAVES, LIGHT, and the ELECTROMAGNETIC SPECTRUM
What is a wave? For our purposes, we will think of a wave as something that travels
from one place to another. The shape of a wave is usually something like alternating
bumps and valleys: first a bump, then a valley, then a bump, and so on. Here is a cartoon
of a wave going by on the surface of a body of water, moving to the right.
What am I doing here?
I'm a hummingbird!
As you may have noticed, there is a hummingbird hovering just above the surface of the
water in the middle of one of the "valleys" (between two bumps).
 If our hummingbird continues to hover in that specific place in space (so that it
does not move), what will happen to it as the wave moves? (Think about it! This
may seem obvious, but it is an important point about the behavior of waves.)
You probably concluded that the hummingbird would get wet. Now for the important
point about waves…
 Two students are arguing. Student #1 says that waves move back and forth in a
zigzag type motion. Student #2 says that unless something gets in the way waves
move (for the most part) in straight lines. "The shape of the wave," Student #2
says, "is not the same as the direction that the wave is going." Based on your
ideas about the wave above, which student do you and your hummingbird agree
with? Did that wave move in a straight line or in a zigzag?
Among the other things that waves do, they carry energy. Our poor hummingbird was
smacked with the energy of a passing water wave.
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 Look at the two waves below. Imagine that you saw the surface of the sea on a
day when it appeared like wave #1 and on a day when it appeared like wave #2.
On which day would you say the sea had more energy? (assume waves are the
same height)
Wave #2
Wave #1
Hopefully by now you have concluded that waves travel in straight lines, not zigzags, and
that waves in which the crests are close together seem to carry more energy than long
waves. If you are having trouble believing either of these things, you are not alone. They
are two of the most important and most misunderstood properties of waves.
Light Waves:
Light travels in waves. Remember, this does not mean that light travels along a zigzag
path. It means that light travels in "packages" that are shaped like waves (we call them
waveforms).
We usually think of a wave as something that goes by in lumps and bumps. First one
bump goes by, then another, and then another. We don't notice light going by that way
because the bumps are so small and they go by so fast. When a wave of orange light
reaches our eyes, for example, there are half a million bumps crammed into every foot,
and it only takes a nanosecond (one billionth of a second) for those half a million waves
to go by. Still, it is precisely the size of those waves, so small that we can fit half a
million of them into one foot (or more precisely 1.5 million into each meter) that tells our
eyes that we are looking at orange light and not blue light (2 million waves per meter) or
red light (1.3 million waves per meter).
Imagine you have two lights that are equally bright, a blue light and a red light. The blue
light packs 2 million waves into each meter. The red light only gets 1.3 million waves
into each meter.
 We call the length of a wave the “wavelength.” Which one has longer waves, the
blue light or the red light? Explain your reasoning.
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 Which one carries more energy, the blue light or the red light? Explain your
reasoning.
 It turns out that the blue light and the red light move with the same speed (three
hundred million meters per second). We call the number of waves that pass each
second the frequency. Which one sends more waves past your eye per second
(a.k.a. which has a higher frequency)? Explain your reasoning.
The visible light spectrum
When we compare blue light to red light we see that blue light has a shorter wavelength,
higher frequency, and carries more energy for the same amount of brightness (red light
has a Longer wavelength, Lower frequency, and Less energy – the “L”s go together).
Still, what’s the fun of knowing that if you don’t understand color? It turns out that most
of us have eyes that detect three colors of light: Red, Green, and Blue. Some people
detect fewer colors (they have partial color blindness) but nobody detects more.* Every
other color you have perceived in your life has been a mixture of those three colors of
light. Every color on a computer monitor is a combination of red, green, and blue dots.
ACTIVITY #1: Open Microsoft Word to a new document page and look at the white
page on the screen with a magnifier. See all of the tiny pretty red, green, and blue dots?
Cool, huh?
ACTIVITY #2
Find a computer and go the following web site (this site is also on our “links” page on the
IDS web site):
http://mc2.cchem.berkeley.edu/Java/emission/Java Classes/emission.html
 What happens when you have red and green at the maximum intensity?’
Technical detail: we can still see a single wavelength of light, even if it has a wavelength somewhere
between green and red. When we see that wavelength, it triggers the receptors in our brain for both green
and red, but not as strongly as if we saw only green or only red light. The curious thing is that we can't
distinguish between a yellow light that is all one wavelength, and a mixture of red and green light that
appears to be the same shade of yellow. We also see violet light even though it has a shorter wavelength
than blue light. Our eyes are not very sensitive to violet light, however, and violet light has to be very
bright for us to perceive it as being equally bright with, say, green light.
*
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 What happens when you have green and blue at the maximum intensity?
 What happens when you have red and blue at the maximum intensity?
 What happens if you have all three colors at the maximum intensity?
 What combination of colors produces orange?
ACTIVITY #2B: Somewhere around the room find a “light box” that emits all three
colors of light. Don’t pick up the light box; they fall apart easily. Move the mirrors
around to make different mixtures of red, green, and blue light (if you want to block one
of the colors of light, try putting a hand or a sheet of paper in front of it).
 What color do you see when you mix red and green light:

What color do you see when you mix green and blue light:
 What color do you see when you mix red and blue light:
 What color do you see when you mix red, green, and blue light:
We say that red, green and blue are the primary colors of light. When we see all three
colors mixed equally, our eyes perceive that as “white light,” so you can think of white
light as an equal mixture of red, blue, and green.
 Before you go to the next web page, imagine that you have some white light. If
you could absorb all of the blue light from it, what color would remain?
 Before you go to the next web page, imagine you have some white light. If you
could absorb all of the red light from the white light, what color would remain?
 ACTIVITY #3:
Next go to the following site and check your answers:
http://mc2.cchem.berkeley.edu/Java/absorption/Java Classes/absorption.html
 Describe absorption:
ACTIVITY #4: Go to the next web page:
http://mc2.cchem.berkeley.edu/Java/single/Java Classes/single.html
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By playing with the controls on this web site, create a definition for a filter. How is an
optical filter different than something like a “water filter”?
 If you are looking through a red filter at a white object, what color will it appear?
 If you are looking at a yellow object through a red filter what color will it appear?
 If you look at a blue object through a red filter, what will you see?
In Summary: Absorption, reflection, and transmission
When light encounters a substance, there are three things that can happen, and sometimes
they all happen at once.
1. The light can be reflected which means that it bounces off. It changes
direction, but aside from that it is pretty much unchanged. A mirror is very
smooth and it reflects light all in the same direction. A piece of sandpaper is
rough and it scatters light in all directions. Most objects are somewhere in
between. Most of the objects we see in our everyday world reflect light to our
eyes—that is why we see the objects. (Some people have the misconception
that in a totally dark room, your eyes will eventually adjust so that you can see
objects in the room. This is not true! If there is no light to reflect off an
object, we would not see the object!)
2. Light can be absorbed which means that the energy in the light is absorbed by
the substance. Something that absorbs some colors (or wavelengths) of
visible light is called a pigment and it is what we use to make paint. When
light is absorbed, the light is gone but the energy remains in the substance in
another form. (Hint of things to come: the energy usually comes back out!)
3. Light can be transmitted which means that it passes through the substance. A
window is clear because visible light is transmitted. Stained glass appears
brightly colored because some colors (or wavelengths) are absorbed and others
are transmitted. Something that transmits some wavelengths but not others is
called a filter.
Check your understanding with the following questions:
 Imagine that white light were to hit a substance that absorbed all of the blue light
so that a mixture of red and green light was reflected. Read that sentence again
and ask questions if you don’t understand. When your eye detects the red and
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green light that is reflected, what color would your eye see? What color would
you say this substance is?
 Imagine that white light were to hit a substance that absorbed all of the green
light so that a mixture of red and blue light was reflected. When your eye detects
the red and blue light that is reflected, what color would your eye see? What color
would you say this substance is?
A substance that absorbs some colors and reflects others is called a pigment. We say that
the three primary colors of pigment (or paint) are yellow, cyan, and magenta. (In primary
school you probably learned that the primary colors of paint were red, blue, and green, but
you never could get that cool magenta or turquoise color, could you?)
ACTIVITY #5: Find some colored paper (pigments) and filters (translucent plastic).
You should have magenta, yellow, and cyan sheets of paper and at least a red and blue
filter. (Our cyan paper is not truly cyan, but it is close!.)
 White light is hitting each of your sheets of paper. Think of which two colors are
reflected by each of them:
 Cyan
 Magenta
 Yellow
 The red filter only lets red light through. How will the three sheets of paper
appear through the red filter? Make a prediction and then place the three sheets of
paper so that they are overlapping but you can see all of them. Place the red filter
over them and record your observations. Do you understand why you see what
you see?
 The blue filter only lets blue light through. Which two sheets of paper will look
the same through the blue filter? How will the other one appear? Make a
prediction and then repeat the experiment with the blue filter. Record your
observations.
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If you understood the previous sections on visible light, you have understood a great deal.
Light does not just come in one wavelength (color). There is a whole spectrum of colors.
The spectrum is the complete collection of all possible wavelengths. When we separate
all of the different wavelengths that are hidden in white light, we see the spectrum as a
rainbow.
ACTIVITY #6: Put on a pair of "rainbow glasses" and try not to look silly. The rainbow
glasses contain diffraction gratings, which separate white light into a spectrum the same
way that prisms do.
1. Look at a white light (use an incandescent light - not a fluorescent light) through
the glasses. You should see lots of rainbows stretched out in many directions.
Ask your instructor to increase or reduce the energy that the light is producing.
 When the energy is increased, what happens to the brightness of the light?
 The total amount of light increases when the brightness is increased. Now
think about the fraction of the light that appears as different colors. When
the energy is increased, what happens to the relative amount of blue light?
What happens to the relative amount of red light?
The incandescent bulb produces light simply because it is hot.
2. Some other light sources (fluorescent bulbs, neon lights, sodium lights) produce
light through specific atomic changes. Look at a neon light, sodium light, or other
chemical gas light through the glasses.
 What do you notice about the spectrum produced by neon light, sodium
light, or other chemical gas light?
 How would you describe the difference between the spectrums produced
by the incandescent bulb and the chemical gas bulb? The spectrum of the
incandescent bulb appears to be more… more what?
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 Think about rainbows that you have seen in the sky. These are the
spectrum of the Sun. What does this tell you about the spectrum of the
Sun? Is the spectrum more similar to the incandescent bulb or the gas
bulb?
We are progressing toward an understanding of the Earth’s “greenhouse effect”. The
process involves the transmission, absorption and emission of energy in the form of
waves. Please let us know if you do not understand these properties.
from : http://www.lbl.gov/MicroWorlds/ALSTool/EMSpec/EMSpec2.html
Electromagnetic waves
Most of the waves we are familiar with, such as waves in water and sound waves, require
a medium (or substance) to travel through. However, light is part of a spectrum of
electromagnetic radiation that will travel through a vacuum (no substance). Electric and
magnetic "fields" can carry waves the same way the surface of a body of water can. Light
travels along as "bumps and valleys" of electrical "pushes and pulls." Honest.
When this property of light was discovered, it immediately raised a question. We see
electromagnetic waves (EM waves) with wavelengths between 450 nanometers (blue)
and 700 nanometers (red). We call them light. Are there EM waves with longer
wavelengths? Shorter wavelengths? The physics of electricity suggested that there
would be, but we could not see them.
We now know that there are EM waves with wavelengths thousands of times shorter than
blue light (and thus energy thousands of times greater than light). There are EM waves
with wavelengths longer than light, too. Our eyes don't detect them, but they are
important in nature and we use them in technology.
ACTIVITY #7: Using the EM chart above rank order the list of wave types in order (1
means largest wavelength) from those types that have the longest wavelengths to those
that have the shortest wavelengths
microwaves
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radio waves
gamma rays
hard (used to study rocks) x-rays
soft (medical) x-rays
infrared
ultraviolet
visible light
From the same list as the previous question, rank the types of energy according to the
amount of energy carried by each type of wave in the space below (1 means the greatest
energy):
Recall that waves that have a short wavelength have the highest energy. This is the reason
that there are limits to amount of x-rays a person should be exposed to during a certain
period of time (this is mostly an issue for x-ray technicians rather than the patients).
Fortunately for us, the Sun does not produce a lot of gamma rays and x-rays. Most of the
gamma rays and x-rays that come to the Earth from elsewhere in the universe are
absorbed in the far upper atmosphere (above the troposphere). The small amount of high
energy EM radiation reaching the Earth is a good thing because otherwise life on Earth as
we know it would not be possible.
A short review:
 As electromagnetic radiation from the Sun arrives at the Earth, what are the three
things that can happen to this energy?
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