Why the sky is often red at sunset. Dust lets more of red and yellow light through than of any other color, and for this reason there is much red and yellow in the sunset. Just before the sun sets, it shines through the low, dusty air. The dust filters out most of the light except the red and yellow. The red light and yellow light are reflected by the clouds (for the clouds are themselves "white"; that is, they reflect all the colors that strike them), and you have the beautiful sunset clouds. Sometimes there is a purple in the sunset, and even green. But since the air itself is blue (that is, it lets mostly blue light go through), it is easy to see how this blue can combine with the red or yellow that the dust lets through, to form purple or green.
But we could not have sunset colors or all the colors we see on earth, if it were not that the sunlight is mostly white—that it contains all colors. And that, too, is why we can have a rainbow.
How rainbows are formed. You already know fairly well how a rainbow is formed, since you made an imitation of one with a prism. A rainbow appears in the sky when the sun shines through the rain; the plain white light of the sun is divided up into red, orange, yellow, green, blue, indigo, and violet. As the white light of the sun passes through the raindrops, the violet part of the light is bent more than any of the rest, the indigo part is bent not quite so much, and so on to the red, which is bent least of all. So all the colors fan out from the single beam of white light and form a band of color, which we call the rainbow.
Fig. 95. Explain why the breakers are white and the sea green or blue.
How we can tell what the sun and stars are made of. When a gas or vapor becomes hot enough to give off light (when it is incandescent), it does not give off white light but light of different colors. An experiment will let you see this for yourself.
Experiment 53. Sprinkle a little copper sulfate (bluestone) in the flame of a Bunsen burner. What color does it make the flame?
Copper vapor always gives this greenish-blue light when it is heated. The photographer's mercury-vapor light gave a greenish-violet glow. When you burn salt or soda in a gas flame, you remember that you get a clear yellow light. By breaking up these lights, somewhat as you broke up the sunlight with the prism, chemists and astronomers can tell what kind of gas is glowing. The instrument they use to break up the light into its different colors is called a spectroscope, and the band of colors formed is called the spectrum. With the spectroscope they examine the light that comes from the sun and stars and by the colors of the spectra they can tell what these far-distant bodies are made of.
Application 39. If you were going to the tropics, would it be better to wear outside clothes that were white or black?
Application 40. A dancer was to dance in a spotlight on the stage. The light was to change colors constantly. She wanted her robe to reflect each color that was thrown on it. Should she have worn a robe of red, yellow, white, green, or blue?
Application 41. If you looked through a red glass at a purple flower (purple is red mixed with blue), would the flower look red, blue, purple, black, or white?