Rainbow
Rainbow, an arc or circle of light of different colors, caused by the refraction and reflection of sunlight by drops of water. Rainbows can be seen only when the sun shines on the drops from behind the viewer. Rainbows typically occur after a brief thundershower when the sun is low in the sky. They can also occur in the mist of a waterfall and in the spray from a garden hose.
Sunlight is composed of all the colors that appear in a rainbow: violet, blue, green, yellow, orange, and red. Each of these colors has a different range of wavelengths. As a result, each color is refracted (bent) at a different angle when sunlight enters a raindrop. Red is bent the least, violet the most. When the colors strike the far side of a raindrop, they are reflected. As they leave the raindrop and enter the air again, they are again refracted.
A rainbow is formed by light scattered by large numbers of water droplets. Although each droplet separates sunlight into colors, the different colors in a rainbow come to the eye of a viewer from different groups of droplets. A rainbow is curved because it can appear along any line of sight that forms a specific angle in relation to the direction of light from the sun. Rainbows seen in the spray from a hose often form a complete circle. However, the rainbow that appears after a rain shower forms an arc instead of a complete circle. This is because there are no raindrops below the level of the horizon to scatter sunlight back to the viewer. When the sun is high in the sky, no rainbow can be seen.
There is an old tradition that a pot of gold lies under the end of every rainbow, and that the first person to reach the spot may secure the treasure. Unfortunately, it is impossible to reach the end of a rainbow—when the observer moves he sees a new rainbow that appears to be the same distance away.
Kinds of RainbowsIn the primary rainbow, which is the type most often seen, red forms the outermost band, and then follow orange, yellow, green, blue, and violet. A fainter secondary rainbow is often seen outside the primary rainbow. In it, the colors are reversed. Violet appears on the outside, red on the inside.
The secondary rainbow is created by light rays that enter the bottom of a raindrop. When they strike the far side of the raindrop, they are reflected to the top, from where they are reflected back toward the viewer. Thus they are reflected twice, while rays of primary rainbows are reflected only once.
Intersecting rainbows are sometimes seen after a rain shower when sunlight is reflected from a body of water. The reflected rays form one rainbow and direct rays from the sun form another. The rarely seen lunar bow, or moonbow, is formed by sunlight reflected from the moon.