Blue Sky: How do Particles in the Atmosphere Cause the Sky to Look Blue and the Sun to Look Yellow?
How do particles in the atmosphere cause the sky to look blue and the sun to look yellow?
- Masking tape
- Sheet of typing paper
- Box with sides at least 8 1/2 × 11 inches (21.3 cm × 27.5 cm)
- 7-ounce (210-ml) clear plastic glass
- Tap water
- Thick, milky, dishwashing liquid
- Tape the typing paper to the inside bottom of the box.
- Set the box on its side on a table.
- Fill the glass three-fourths full with water.
- Set the glass inside the box near the front edge.
- Darken the room and use the flashlight to direct a light beam through the center of the glass.
- Observe the color of the light on the white paper behind the glass and the color of the water in the glass.
- Add one drop of dishwashing liquid to the water and stir.
- Again, place the glass inside the box and shine the light beam through the center of the glass.
- Observe the color of the light on the white paper behind the glass and the color of the soapy water in the glass.
Light passing through the water produces a white spot of light on the paper and makes the water look bright, but it has no color. Light passing through the soapy water produces a yellow spot on the paper and gives the water a bluish appearance.
Light is a form of energy that travels in waves like water waves. The color of light depends on its wavelength (distance from the top of one wave to the top of the next wave). The order of the seven colors of the rainbow are red, orange, yellow, green, blue, indigo, and violet. These colors are called the visible spectrum and are in order from the longest to the shortest wavelength, red being the longest and violet the shortest.
Light from the flashlight, like light from the sun, looks white, but is actually a combination of all the rainbow colors. When the light waves from the flashlight pass through the soapy water, they come in contact with small soap particles and the shorter blue wavelengths of light and are scattered in different directions. You see more blue light, so the soapy water looks blue.
This same kind of scattering of blue light waves occurs as sunlight passes through the atmosphere, the blanket of air surrounding the earth. Air molecules (the smallest unit of substance that still is that substance) in the atmosphere are just the right size to scatter the shorter wavelengths of light, mostly the blue light rays. No matter which direction you look, blue light waves from the sky come to your eyes. This is what makes the sky look blue. The combination of the remaining light waves produces a yellow color. This is what causes the yellow spot on the paper and also causes the sun to appear yellow.
- At sunset and sunrise the sun is farther away from you than when it is directly overhead. During this time, the sunlight has to pass through more of the atmosphere to reach you. This longer journey through the air means more of the shorter waves of light are scattered. Few reach your eye. The longer wavelengths of yellow, orange, and red light waves are scattered less, so you see more of these colors causing you to see a flaming sky. Demonstrate this effect by increasing the number of soap particles in the water. Repeat the original experiment, adding one drop of dishwashing liquid at a time until 15 drops have been added. Compare the colors of the light that appear on the paper after each drop of soap is added.
Science Fair Hint: Find out how the sky's color can be used as a weather predictor. How accurate are weather proverbs such as: "Red sky in the morning, sailors take warning; red sky at night, sailors' delight"? Display this and other weather proverbs. Include information about the accuracy of their predictions. One source for this information is on pages 53-56 of The Weather Companion by Gary Lockhart (New York: Wiley, 1979).