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IDS 101

Light, Color, and the Greenhouse Effect

Imagine that you have a light source and some way to detect the intensity of light at

various distances. If you increase the distance of the detector from the light bulb, the

intensity of the light decreases dramatically. From lab measurements of this type and

knowledge of the energy radiating from the Sun, we can predict the average temperature

of the surface of various planets.

As you may know, Venus is the second planet from the Sun, while the Earth is the third

planet out from the Sun. Since Venus is closer to the Sun, it is reasonable that the average

surface temperature of Venus should be higher than the average surface temperature of

Earth. If we do some calculations we find that the temperature of Venus’s atmosphere

should be around 67C. This compares to the average temperature of Earth’s atmosphere

at about 15C. This sounds logical, but the temperature at the surface of Venus is about

460C! The purpose of this module is to help you understand how the gases in our

atmosphere help drive the hydrologic cycle.

The atmosphere of Venus is about 96% carbon dioxide (CO2). This gas has a major role

in creating what we term a ―greenhouse effect.‖ Before we discuss the role of CO2 in both

the Earth’s and Venus’s atmosphere, we need to understand more about electromagnetic

radiation, waves, reflection, transmission, and most importantly about absorption.

An aside: What is even more amazing is that about 80% of the solar radiation arriving at Venus is reflected

back into space. (For comparison, about 29% of the energy arriving at the Earth is reflected back into

space.) The reason for this large reflectance value for Venus is the density of the atmosphere of Venus. The

atmosphere of Venus is 90 times as dense as the Earth’s atmosphere. (Is the air pressure on Venus higher or

lower than on Earth?)

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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. Very good. 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.

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)

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Wave #1

Wave #2

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 (take a guess) the wavelength. Which one has longer

waves, the blue light or the red light? Explain your reasoning.

Which one carries more energy, the blue light or the red light? Explain your

reasoning.

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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?

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 a blank white page in Word. Look at the computer screen using a

magnifying glass. See all of the pretty red, green, and blue dots? Cool, huh?

ACTIVITY #2

Find a computer and go the following web site:

http://chemistry.beloit.edu/Stars/pages/colormix.html

Click to "RGB Color Mixing."

* 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 red and green at the maximum intensity?

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 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?

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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.

ACTIVITY #3:

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?

Next go back to the same initial site, click on "Multiple Filter Absorption", and check

your answers:

http://chemistry.beloit.edu/Stars/pages/colormix.html

What is a definition for absorption?

ACTIVITY #4: Go back to the main web page and click on Single Filter Absorption:

By playing with the controls on this web site, create a definition for a filter.

If you are looking through a red filter at a white object, what color will the object

appear?

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If you are looking at a yellow object through a red filter what color will the object

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.

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

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?

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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.

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

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all of the different wavelengths that are hidden in white light, we see the spectrum as a

rainbow.

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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?

Go the web site listed below.

http://chemistry.beloit.edu/Stars/EMSpectrum/index.html

You should see the graphic below. The short bars are links to other pages with

information about the various wavelengths. Use this information to complete the

questions below:

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: Consult the chart of the electromagnetic spectrum on the previous page

and try to identify the following kinds of EM waves. See if you can fill in this table.

Name of EM wave Wavelength Energy

Gamma

X-ray

Ultraviolet (UV)

Visible About 500 nm Medium

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Infrared

Radio

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?

Recall that energy can be reflected, absorbed and/or transmitted. The climates of the

Earth and Venus are dependent on the amount of reflection, absorption, and transmission

of the Sun’s energy. Let’s study these ideas a little more….

We use the term albedo to describe the amount of radiation that the Earth reflects back

into space. On the first page of this module you read that about 29% of the Sun’s energy

reaching the Earth is reflected back into space. (Imagine if no energy were reflected, it

would have been difficult for the astronauts on the moon to see the Earth!)

The table below is the albedo values for different types of earth surface:

Earth Surface Type Average

Albedo

Forests 15%

Agricultural land 20%

Deserts 28%

Snow and ice cover 80%

Ocean (<70 º latitude) 3.8%

Ocean (>70 º latitude) 9.2%

Clouds 50%

As you might expect, snow and ice reflect a lot of incoming radiation, while

forests do not reflect nearly as much radiation. Agricultural land reflects more

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radiation than forests, so what effect have people had on the Earth’s albedo by

cutting forests and growing crops in the same location?

Ice and snow reflect more radiation than ocean water. If large ice sheets melt and

there is more ocean water surface, how will the albedo of the Earth change?

If there is an increase in the Earth’s albedo, what will happen to the temperature

of the Earth?

What happens to the 71% of the energy that does not reflect back into space?

One of the types of radiation you labeled under the Electromagnetic Waves section above

was ultraviolet radiation (or UV). UV radiation is often called ultraviolet ―light‖ even

though we can’t see it. For the sake of accuracy, we should try to use the word ―light‖ to

describe only what we can see. Despite what you may have seen with "black lights" that

are commercially available, you cannot see ultraviolet waves. The violet light that we see

coming from "black lights" is light with a wavelength that is not quite short enough to

really be ultraviolet. A black light makes ultraviolet waves as well, but you can't see

them.

Look back at your table of different kinds of EM waves (p. 9) with different

energies. Do you think a molecule could absorb some UV waves and then give

off gamma rays or x-rays? Explain your reasoning. (Hint: if Keith gave you a

dollar could you turn around and give Bob a million dollars?—Bob likes the

idea!)

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Could a molecule absorb some UV waves and then give off visible light or

infrared light? Explain your reasoning. (Think about that dollar.)

Activity #8

Ask one of your instructors for help with one of the UV sources. CAUTION! These

are high intensity UV sources and they are very different from ordinary black lights. Do

not look directly into them when they are in use!

Shine a UV source on a dull patch of the wall (not a shiny surface and not a

"bleached" surface such as a piece of paper or your socks). Do you think you can

see ultraviolet waves? (Could this just be a very dim ordinary light?)

Shine a UV source on one of the wondrous and very cool rocks of science. Does

it look the same as the wall? What do you suppose is happening to the UV

radiation that is being absorbed by the rock?

If we had a source of infrared radiation, could we shine the IR on the rock and get

the same result? Why? Why not?

When infrared radiation (IR) shines on our skin, we feel it as heat. Heat can move from

one place to another by conduction, by convection, or by radiation. When heat travels by

radiation it is traveling in the form of infrared radiation.* When we feel "the warmth of

the Sunshine" we are feeling infrared waves that reach the Earth after traveling a hundred

million miles. Another example is when we feel warm from a campfire even when the air

around us is cold.

* Technical note: Some textbook authors separate infrared waves from short wavelength radio waves, in

which case one has to say that radiated heat travels as infrared and/or radio waves. For the purposes of

studying weather and climate, it is sufficient to call both of these forms by the name "infrared.‖ Still, you

may be interested to know that every time you cook something in a "microwave" oven, you are heating it

up with very intense radio waves.

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Whenever we feel heat radiated by anything, we are feeling infrared waves. Any object

that is warmer than its surroundings will radiate IR. Any object that is cooler than its

surroundings will absorb more IR than it radiates.

Ask for a special type of thermometer when you reach this point. This special

thermometer can measure our skin temperature without touching us. How did this

thermometer measure our temperature without touching our skin?

Actually, the atmosphere absorbs or reflects much of the infrared radiation that

reaches the Earth, so much of the infrared radiation that heads our way does not

make it down to the surface of the Earth. If the atmosphere absorbs the IR, what

happens to the atmosphere?

An important idea most of the "solar energy" that reaches the surface of the

Earth from the Sun is in the form of VISIBLE LIGHT. (Infrared trails just behind.)

Remember, visible light is higher in energy than infrared.

Key question: When objects on the surface of the Earth absorb visible light,

can they turn around and give off ultraviolet waves? X-rays? (HINT: Some

objects can give off gamma waves even if they don't absorb anything, but what

we are really asking here is whether the absorption of some visible light would

cause something to be able to give off x-rays or UV waves.)

When objects on the surface of the Earth absorb visible light, can they turn

around and give off infrared waves? Explain your reasoning.

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In the summer you may have noticed that when you touch a dark colored object,

the object feels very hot. If very little IR reaches the Earth’s surface, why is the

object hot?

If 71% of the Sun’s energy is not reflected back into space, it is mostly absorbed by the

Earth’s surface. Objects on the Earth’s surface re-emit that radiation in the form of IR.

We call this ―black body radiation‖. When the IR is re-emitted by the Earth, gases in the

atmosphere absorb most of that IR and the atmosphere becomes warmer.

If your car has been parked for a couple of hours with the windows closed you

will find that when you get into your car that the temperature inside the car is

higher than the outside temperature. Why is the inside of your car warmer than

the outside air?

Gardeners use greenhouses to provide a warmer, lighted environment for plants to

grow. Draw a picture of how a greenhouse works.

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In what ways is the Earth similar and different than a greenhouse?

The gases in the atmosphere act like the glass in a greenhouse, keeping the infrared

radiation from escaping, so the temperature inside increases. The specific gases that

absorb IR are carbon dioxide, methane, water vapor, and chorofluorocarbons (cfc’s such

as ―Freon‖). The first three of these gases are natural gases, but the fourth is a human

compound that was used primarily in air conditioners and refrigerators until the last few

years.

The arc below is intended to be the surface of the Earth. Create a diagram that illustrates

and explains how the greenhouse effect increases the temperature of the atmosphere.