I Didn’t Know That . . .

Q: How is a rainbow formed?

A: Rainbows occur because light waves are refracted--bent--upon entering and leaving a drop of water, just as a beam of sunlight is refracted as it passes through a prism. For a person to see a rainbow, the sun must be behind him, and the angle between the path of a light ray from the sun to the raindrop and the path from the drop to the eye must be about 41 degrees. For red light, which is bent the most, the angle is 42 degrees, while for violet light, which is bent the least, the angle is 40 degrees, according to the New Book of Popular Science. The observer will receive the impression of color from a vast number of drops which, from his viewpoint, form an arc in the sky.

A secondary, or outer, bow of the rainbow is formed by light that is reflected twice inside each raindrop rather than once. The order of colors is reversed and the colors are more faint..