Red Sky, Blue Sky

Objective: To illustrate how the gases in the atmosphere scatter some wavelengths of visible light more than others.

Materials: Aquarium Stirrer Flashlight Opaque card with hole Water Milk Eye dropper Dark room Procedure: 1. Fill the aquarium with water and set up the demonstration as shown in the illustration. 2. Add a few drops of milk to the water and stir the water to mix the two liquids. You may have to add more drops to achieve the desired color change effect. Refer to the discussion for more information. 3. Darken the room and turn on the flashlight. 4. Observe the color of the light coming from 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

colors out of the direct path of sunlight and leave the other colors to travel straight through. This makes the Sun look yellow-white and the rest of the sky blue. This effect is accentuated when the Sun is low in the sky. At sunrise and sunset, sunlight has to penetrate a much greater thickness of atmosphere than it does when it is overhead. The molecules and dust particles scatter almost all of the light at sunrise and sunsetQblue, green, yellow, and orangeQ with only the red light coming directly through to your eyes; so, the Sun looks red. Caution: Never stare directly at the Sun. In this demonstration, the suspended particles of milk scatter the light like the molecules in Earth's atmosphere. When the flashlight beam is viewed directly through the water, the blue wavelengths of light are scattered away from the beam of light, leaving it yellowish. Increasing the amount of milk simulates smog and the Sun will look red. Viewing the water from the