Light, Color, and the Greenhouse Effect

Transcription

1 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 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 gases in our atmosphere help drive the hydrologic cycle. The atmosphere of Venus is about 96% carbon dioxide (CO 2 ). This gas has a major role in creating what we term a greenhouse effect. Before we discuss the role of CO 2 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?) 1

2 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) 2

3 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. 3

4 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: 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. 4

5 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? 5

6 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: 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? 6

7 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? 7

8 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 8

9 all of the different wavelengths that are hidden in white light, we see the spectrum as a rainbow. 9

10 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? 10

11 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. 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 11

12 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 12

13 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!) 13

14 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. 14

15 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. 15

16 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. 16

17 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. 17