IDS 102
Trace Gases in the Atmosphere
“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 67°C. This compares to the average temperature of Earth’s atmosphere at about 15°C. This sounds logical, but the temperature at the surface of Venus is about 460°C! The purpose of this module is to help you understand how the greenhouse effect has made the Earth a place in which life can exist, while on Venus, the same process produces temperatures hot enough to melt lead at the surface.
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?)
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.
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).
Ø 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…
Ø 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.
Ø 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)
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
To understand the greenhouse effect on Earth, we must first understand the concepts of emission, absorption, and reflection. To understand these topics we will use light. 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.
Ø 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.
Ø Which one carries more energy, the blue light or the red light? Explain your reasoning.
Ø 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 Microsoft Word to a new document page and look at the white page on the screen with a magnifier. See all of the 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
Ø What happens when you have red and green at the maximum intensity?’
Ø What happens when you have green and blue at the maximum intensity?
Ø What happens when you have red and blue at the maximum intensity?
Ø What happens if you have all three colors at the maximum intensity?
Ø 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).
Ø What color do you see when you mix red and green light?
Ø What color do you see when you mix green and blue light?
Ø What color do you see when you mix red and blue light?
Ø 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.
Ø 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?
Ø 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
Ø What is a definition for absorption?
ACTIVITY #4: Go to the next web page:
http://mc2.cchem.berkeley.edu/Java/single/Java Classes/single.html
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”?
Ø If you are looking through a red filter at a white object, what color will it appear?
Ø If you are looking at a yellow object through a red filter what color will it appear?
Ø 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:
Ø 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?
Ø 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!.)
Ø White light is hitting each of your sheets of paper. Think of which two colors are reflected by each of them:
à Cyan
à Magenta
à Yellow
Ø 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?
Ø 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 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.