Unit 1 The Atmospheric Filter Introduction regions of it. Relying on the radiation that reaches Earth's surface is like listening to a Earth's atmosphere is essential to life. This piano recital with only a few of the piano's ocean of fluids and suspended particles keys working. surrounds Earth and protects it from the hazards of outer space. It insulates the Unit Goals inhabitants of Earth from the extreme temperatures of space and stops all but the • To demonstrate how the components of largest meteoroids from reaching the Earth's atmosphere absorb or distort surface. Furthermore, it filters out most incoming electromagnetic radiation. radiation dangerous to life. Without the • To illustrate how important observations atmosphere, life would not be possible on above Earth's atmosphere are to Earth. The atmosphere contains the oxygen astronomy. we breathe. It also has enough pressure so that water remains liquid at moderate Teaching Strategy temperatures. The following activities use demonstrations Yet the same atmosphere that makes life to show how the components of Earth's possible hinders our understanding of atmosphere filter or distort electromagnetic Earth's place in the universe. Virtually our radiation. Since we cannot produce all of only means for investigating distant stars, the different wavelengths of electromagnetic nebulae, and galaxies is to collect and radiation in a classroom, the light from a analyze the electromagnetic radiation these slide or filmstrip projector in a darkened objects emit into space. But most of this room will represent the complete electro- radiation is absorbed or distorted by the magnetic spectrum. A projection screen atmosphere before it can reach a ground- represents Earth's surface and objects based telescope. Only visible light, some placed between the projector and the screen radio waves, and limited amounts of infrared represent the effects of Earth's atmosphere. and ultraviolet light survive the passage With the exception of a take-home project, from space to the ground. That limited all the demonstrations can be conducted in amount of radiation has given astronomers a single class period. Place the projector in enough information to estimate the general the back of the classroom and aim it shape and size of the universe and towards the screen at the front. Try to get categorize its basic components, but there is the room as dark as possible before doing much left to learn. It is essential to study the the demonstrations. entire spectrum rather than just limited 9 Activity Titles Clear Air Activity Objective: To demonstrate that gas molecules in the atmosphere absorb some of the visible light that passes through them. Application: Astronomy, Physical Science Water In The Air Activity Objective: To show that moisture is present in the atmosphere and demonstrate how it absorbs visible light. Application: Astronomy, Meteorology, Physical Science Red Sky, Blue Sky Activity Objective: To illustrate how the gases in the atmosphere scatter some wavelengths of visible light more than others. Application: Astronomy, Meteorology, Physical Science Ultraviolet Absorption Activity Objective: To show that the atmosphere is transparent to low-energy ultraviolet light but not to high-energy ultraviolet light. Application: Astronomy, Environmental Science, Physical Science Particle Pollution Activity Objective: To observe the effects suspended particles of pollution have on the transmission of visible light. Application: Astronomy, Environmental Science, Meteorology Particulate Sampler Activity Objective: To obtain a quantitative measurement of the particulate pollution present in the neighborhood of the students and school. Application: Astronomy, Environmental Science, Meteorology Heat Currents Activity Objective: To show how visible light appears to shimmer, when seen through rising heat currents. Application: Astronomy, Meteorology, Physical Science Day and Night Activity Objective: To demonstrate the effects of day and night cycles on astronomical observations. Application: Astronomy 10 Clear Air Description: Sheets of clear glass are held between a projector and a screen to show that not all light is transmitted by apparently clear materials. Objective: To demonstrate that gas molecules in the atmosphere absorb some of the visible light that passes through them. Materials: Several small sheets of clear glass or Plexiglass Emery paper (fine)–Use on glass. Projector Screen Dark room Procedures: 1. Before handling the glass sheets, smooth sharp edges with emery paper. Skip to the next step if using plexiglass. 2. Hold up one of the sheets so that the Discussion: students can examine it. Ask them to This demonstration provides an analogy for describe what they see. At some point, the light-filtering effects of the atmosphere. one student will say that the glass is The shadow cast by the glass shows that clear. Ask the other students if they although the glass appears to be clear, it agree that the glass is clear. prevents some of the light from reaching the 3. Darken the room and turn on the screen. The glass reflects some light off its projector. front surface and absorbs some of the light 4. Hold the clear glass between the light and that attempts to pass through it. The effect screen. Observe the distinct shadow cast is similar to what happens with Earth's by the edges of the glass and the slight atmosphere. Part of the visible radiation dimness of the light that has passed attempting to reach Earth is reflected by the through the glass. Place an additional atmosphere, particularly by clouds, and part sheet of glass on top of the first and is absorbed and scattered by the gases in observe any difference in the light as it the atmosphere. passes through double-thick glass. Add a third sheet of glass and repeat. For Further Research: 5. While holding the glass in the projector • Why do the edges of a sheet of glass beam, observe if there are any reflections usually appear green? of the light around the room. • Compare this demonstration with the 6. With the room lights back on, observe the following activities: Red Sky, Blue Sky? edge of the glass. Does light pass (page 14) and Resonance Rings (page through the edge? What color is the 44). edge? What causes this color? 11 Water In The Air Description: The presence of water in the atmosphere is demonstrated and its light-filtering effects are shown. Objective: To show that moisture is present in the atmosphere and demonstrate how it absorbs visible light. Materials: Shallow dish or an aluminum pie plate Empty coffee can Ice water Water spray bottle Cloud cutout (white cardboard or foamcore) Projector Projection screen Dark room Procedure: Part 1. Fill the coffee can with ice water and projector and screen and spray. place it in the shallow dish or pie plate. Observe the shadows on the screen cast Observe the outside of the can every by the fine water droplets. minute or two. Water droplets from the Part 3. Simulate how clouds block visible air will begin to condense on the outside radiation by holding up a cutout of a cloud of the can. between the projector and the screen. Part 2. Darken the room and turn on the projector. Place some clean water in the spray bottle. Adjust the spray to a fine Discussion: mist. Hold the bottle between the The first demonstration shows that water is present in the atmosphere. The capacity to hold water is determined by the atmospheric temperature. Warm air can hold more water than cold air. Because the can is chilled by the ice water, the air immediately surround- ing the can cools. Lowering air temperature reduces its capacity to hold water, and so the excess water condenses on the outside of the can. The amount of water in the atmosphere at any one time, expressed as a percentage of complete saturation, is called the relative humidity. Humid air filters out 12 For Further Research: much of the infrared portion of the • Use a sling psychrometer or other humidity electromagnetic spectrum. Since air's measuring device to determine the relative capacity to hold moisture drops with humidity of the atmosphere. Does the temperature, astronomers build infrared absolute humidity in the atmosphere telescopes on high mountain tops where the change with the air temperature? Is there air is much cooler and therefore drier than at a difference in the clarity of the atmo- lower elevations. Infrared telescopes are sphere between warm and cold nights? also carried on airplanes like NASA's • Design an experiment to compare the Gerard P. Kuiper Airborne Observatory from water capacity of warm and cold air. which observations can be taken at altitudes • Obtain black and white pictures of Earth above 12,000 meters. Another good from space and estimate the total cloud location for viewing is Antarctica because of coverage visible. its dry air. Telescopes in space gain an • Is it better to locate an observatory on a even better view of infrared radiation. high or low point above sea level? Why? • Take your class to a science museum that The second demonstration illustrates the has exhibits on atmospheric phenomena. effect of small water droplets on light passage. The third demonstration shows that clouds are very effective filters of visible light. 13 Red Sky, Blue Sky Description: Milky water is used to simulate a sunset and the blue sky. Objective: To illustrate how the gases in the atmosphere scatter some wavelengths of visible light more than others. colors out of the direct path of sunlight and Materials: leave the other colors to travel straight Aquarium through. This makes the Sun look yellow- Stirrer white and the rest of the sky blue. This Flashlight effect is accentuated when the Sun is low in Opaque card with hole the sky. At sunrise and sunset, sunlight has Water to penetrate a much greater thickness of Milk atmosphere than it does when it is Eye dropper overhead. The molecules and dust particles Dark room scatter almost all of the light at sunrise and sunset—blue, green, yellow, and orange— with only the red light coming directly Procedure: through to your eyes; so, the Sun looks red. 1. Fill the aquarium with water and set up Caution: Never stare directly at the Sun. the demonstration as shown in the illustration. In this demonstration, the suspended 2. Add a few drops of milk to the water and particles of milk scatter the light like the stir the water to mix the two liquids. You molecules in Earth's atmosphere. When may have to add more drops to achieve the flashlight beam is viewed directly the desired color change effect. Refer to through the water, the blue wavelengths of the discussion for more information. light are scattered away from the beam of 3. Darken the room and turn on the light, leaving it yellowish. Increasing the flashlight. amount of milk simulates smog and the Sun 4. Observe the color of the light coming from will look red. Viewing the water from the the flashlight. Next, observe the color of the light as it comes directly through the aquarium. Observe the color of the liquid from the side of the aquarium. Discussion: One of the standard "why" questions children ask is, "Why is the sky blue?" Sunlight has all of the rainbow colors: red, orange, yellow, green, blue, and violet. Earth's atmosphere contains molecules of gas that scatter the blue 14 side reveals a very subtle grey-blue hue. • Draw a diagram on a chalkboard or Note: Because of individual color sensitivity, overhead transparency like the one shown some people may not be able to see the below in which you are looking down at bluish hue. Earth from a position far above the North Pole. Measure the difference in For Further Research: atmospheric thickness the Sun's rays must When energized by penetrate to reach each location on sunlight, oxygen Sunlight and nitrogen atoms Earth's surface in the diagram below. Which ray has the greatest distance to in the atmosphere travel through the atmosphere to reach re-emit (scatter) Earth's surface? light in all direc- • Pretend you are standing at each location tions, causing the looking toward the Sun. What color Atom entire atmosphere should the Sun be? above us to be • What is the approximate local time for lighted by sunlight. Scattered Light (re-emitted from atom) each location? Violet light is scattered the most and red light the least (1/10th as much). Because our eyes are not very sensitive to violet light, the sky appears blue. t h g nli u S C Noon D B R otation A arth's Rotati E E o n Sunset Sunrise East North Pole West EARTH Note: The thickness of Earth's atmosphere is exaggerated for graphic purposes. Midnight 15 Ultraviolet Absorption Description: A piece of glass or clear plastic blocks shortwave ultraviolet light. Objective: To show that the atmosphere is transparent to low-energy ultraviolet light but not to high-energy ultraviolet light. Materials: Ultraviolet light ("black light") - broad spectrum (may be available from a high school science laboratory or a rock and mineral collector) Fluorescent mineral (See note in the discussion section.) Window glass Dark room Procedure: 1. Darken the room and turn on the ultraviolet (UV) light. (See Caution at the end of the discussion section.) Direct the UV light's beam onto the fluorescent minerals. Observe the color of the emitted light. 2. Place the glass between the light and the mineral and again observe the emitted light. Discussion: Certain minerals and a variety of other Low-energy ultraviolet light—sometimes substances fluoresce or emit visible light called long-wave UV—penetrates to Earth's when illuminated by ultraviolet light. surface. This low-energy ultraviolet light Fluorescence is a process that exchanges causes Day-Glo paints to give off ultraviolet-light energy for visible-light spectacular colors and white clothing to energy. Photons of ultraviolet light are glow brightly when washed in detergents captured by electrons orbiting the nuclei of that contain fluorescent dyes (advertised as atoms within those materials. The making clothes "whiter than white"). Most of electrons, gaining energy, are boosted to the high-energy ultraviolet light—sometimes excited energy states. The electrons called short-wave UV—is blocked by the eventually release this captured energy as ozone layer in Earth's upper atmosphere. visible light as they return to lower energy Higher-energy ultraviolet light causes skin states. 16 tanning. Extended exposure can lead to Avoid directing the light to reflective eye damage and skin cancer in light-skinned surfaces. Everyone should wear eye people. Skin cancer is rare in dark-skinned protection such as laboratory safety glasses people. or ordinary eye glasses. Although transparent to lower-energy Where to Obtain Ultraviolet Lights and ultraviolet light, glass blocks higher energy Minerals: ultraviolet light. Lotions that are advertised Many science-supply catalogs sell ultraviolet as Sun blockers also block higher energy lights and fluorescent minerals. If you ultraviolet light. When the glass is inserted purchase a light, be sure to obtain a broad between the lamp and the fluorescing spectrum light because it will emit both long material in this demonstration, the and short wave ultraviolet light. Order fluorescence diminishes or stops. Some minerals, such as calcite, fluorite, and materials fluoresce with lower energy franklinite, that fluoresce at short ultraviolet waves as well as the higher wavelengths, long wavelengths, and both energy waves, and any continued long and short wavelengths. If you do not fluorescence is the result of the lower wish to purchase a lamp and minerals, energy waves. check with other schools to see if they have equipment you can borrow. Also check with Ultraviolet light tells astronomers several local rock and mineral clubs. Many things. For example, the local neighborhood collectors have lights and fluorescent of our Sun—within 50 light years—contains minerals and may be willing to come to your many thousands of low-mass stars that glow school to give a demonstration. If ultraviolet in the ultraviolet. When low-mass stars use minerals are not available, experiment with up all their fuel, they begin to cool. Over ultraviolet-sensitive paints or paper. billions of years, the internal heat left over from stellar fusion reactions radiates into For Further Research: space. This leftover heat contains a great • Check recent magazine articles about deal of energy. These stars, called white problems with Earth's ozone layer and dwarfs, radiate mostly ultraviolet light. Until ultraviolet radiation at Earth's surface. astronomers could make observations with Learn what can be done to help protect ultraviolet telescopes in space, they had Earth's ozone layer. very little information about this phase of a • Take a field trip to a science museum that star's evolution. has displays of fluorescent minerals or arrange for a rock and mineral collector to Caution: Do not look into the light emitted bring a fluorescent mineral display to your by the broad spectrum ultraviolet lamp. school. 17 Particulate Pollution Description: Particulate matter from both natural and human made sources obscures the sky. Objective: To observe the effects of suspended particles of pollution on the transmission of visible light. Materials: Stick or book matches Two dusty chalk erasers Flashlight Projector Projection screen Dark room Procedure: and volcanic eruptions, add large amounts of 1. Darken the room and turn on the projector. dust and ash to the air. All these extra 2. While standing in the projector beam, particles in the air not only filter out sunlight, strike a match. Observe the shadows on causing redder sunsets and sunrises, but the screen that are created by the smoke also increase "light pollution." Street, released by the combustion process. residential, and industrial night lighting reflect 3. Smack two dusty chalk erasers together off particles and water vapor in the and observe the shadows caused by the atmosphere to prevent the night sky from chalk dust. being really dark. However, many urban 4. Turn off the projector and turn on the areas have begun to choose street lighting flashlight. Stand against a dark that interferes less with astronomical background and stir up more chalk dust by observations. Because of light pollution, slapping erasers together. Shine the observatories are usually located as far from flashlight's beam on the dust before it urban areas as possible. Survey teams settles. Observe the brightness of the spend several months on candidate dust particles that are in the beam. mountain tops observing weather patterns. They study particulates, wind, humidity, and Discussion: temperature extensively before committing to Dust and dirt in the atmosphere darken the the construction of an observatory. sky during the daytime. These particulates come from both human and natural activities. For Further Research: Exhaust from internal combustion engines • Check reference books to learn about the makes carbon monoxide gas, nitrogen oxide effects of volcanic eruptions on the clarity gas, and carbon (soot) particles. Industrial of Earth's atmosphere. and home heating and forest fires, such as • How could street light fixtures be modified slash and burn agriculture, also contribute to to reduce light pollution? the dust and dirt in the sky. Natural processes, like lightning-caused forest fires 18