Anything white, like a piece of paper, reflects all the light that strikes it; so if all the colors (white light) strike it, all are reflected to your eyes and the object looks white.
You have looked at people under the mercury-vapor lights in photo-postal studios, have you not? The lights are long, inclined tubes which glow with a greenish-violet light. No matter how good the color of a person is in ordinary light, in that light it is ghastly.
Fig. 94. A mercury-vapor lamp.
Go into the kitchen tonight, light a burner of the gas stove, turn out the light and sprinkle salt on the blue gas flame. The flame will leap up, yellow. Look at your hands, at some one's lips, at a piece of red cloth, in this light. Does anything look red?
The reason why nothing looks pink or red in these two kinds of light is this: The light given by glowing salt vapor or mercury vapor has no red in it; if you tried to make a "rainbow" from it with a prism, you would find no red or orange color in it. A thing looks red when it absorbs all the parts of the light that are not red and reflects the red light to your eyes. If there is no red in the light to reflect, obviously a thing cannot look red in that light.
When you look through a piece of colored glass, the case is somewhat different. A piece of blue glass, for instance, acts as a sort of strainer. The coloring matter in it lets the blue light through it, but it holds back (absorbs) the other kinds of light. So if you look through a piece of blue glass you see everything blue; that is, only the blue part of the light from different objects can reach your eyes through this kind of glass. Anything that is transparent and colored acts in a similar way.
Why the sky is blue. And that is why the sky looks blue. Air holds back all colors of light except blue; that is, it holds them back a little. A room full of air holds the colors back hardly at all. A few miles of air hold them back more; mountains in the distance look bluish because only the blue light from them can reach you through the air. The hundred or more miles of air above you hold back a considerable amount of the other colors of light, letting through much more of blue than of any other color. So the sky looks blue; that is, when the air scatters the sunlight above you, it is chiefly the blue parts of the sunlight that it allows to reach your eyes.
Why bodies of water look green in some places and blue in others. Water acts in a similar way, but it lets the green light through instead of the blue. A little water holds back (absorbs) the other colors so slightly that you cannot notice the effect in a glass of water. But in a swimming tank full of water, or in a lake or an ocean, you can notice it decidedly when you look straight down into the water itself.
When you look at a smooth body of water at a slant on a clear day, the blue sky is reflected to you and the water looks blue instead of green. And it may even look blue when you look straight down in it if it is too deep for you to see the bottom and the sky is reflected from the surface.