Sunrise-Sunset Demo

I use the demo as we begin our study of the atmosphere because it seems that the concept of "wavelengths" comes up so often during the unit . . . For example, topics such as the greenhouse effect and the absorption of UV by ozone molecules both involve an understanding of wavelengths.

An overhead projector with screen, a 1-gallon jar, a small (1/8th tsp.) measuring spoon, a packet of non-dairy coffee creamer, a dark classroom, large spoon or ruler for stirring, flashlight (optional)

1. Fill the jar with water to about 2 inches from the brim.

2. Place the jar onto the overhead projector (no lid). Darken the room. Turn the projector on, projecting the light through the jar of water onto the screen.

3. Have the students observe the "whiteness" of the light projected onto the screen. Remind them that "white light" includes wavelengths of ROYGBIV. Ask them "what is the difference between red light and blue light?". Most of my freshmen know that the red light has a longer wavelength than blue light. Also, point out the colorlessness of the water in the jar.

4. Next, take 1/16th tsp. of the powdered creamer and stir it into the water. Have the students observe the color of light projected onto the screen and the appearance of the water. They may begin to see an orange tint on the screen and a very faint hint of blue in the jar of water.

5. Add another 1/16th tsp. of creamer to the jar of water, stir, and observe. By now the reddish-orange color on the screen should be obvious, and the very faint sky-blue may be apparent in the water, especially near the bottom of the jar.

6. Add another 1/16th tsp. of creamer, stir, and observe. If a flashlight is available, shut off the projector and then shine the flashlight horizontally through the water toward the eyes of the students so they can see the reddish setting or rising Sun.

Light from the Sun includes wavelengths of red, orange, yellow, green, indigo, and violet (ROY G. BIV). The white light from the projector bulb also includes these colors (wavelengths). During the middle of a clear day, the shorter waves, especially blues are scattered by air molecules, dust, and pollutants, whereas the longer waves (ROYG and some of the longer blue wavelengths) are able to pass through the atmosphere. Apparently, air molecules are just the right size to "scatter" the shorter waves (especially blues). As a result, when we look at the atmosphere, we see the "scattered" blue light.

When the sun is rising or setting, sunlight must pass through more atmosphere before reaching our eyes. Even more wavelengths, including the longer blues, the greens, and the yellows are scattered. As a result, when we look at a rising or setting Sun, we see only the reds and oranges, which haven't been scattered, or "filtered out" by stuff in the atmosphere. The powdered creamer has the same effect on light from the bulb as the particles in the atmosphere have on sunlight.

Source for the explanation: The Weather Book by Jack Williams/USA Today, 1992, published by Vintage Books (a division of Randon House)