BOSTON — When it stops raining and the sun comes out, run outside and stand with your back to the sun. If conditions are just right, you'll see a rainbow! (But hurry - rainbows only last for 30 minutes or so.) As you gaze at the rainbow, here are some things you might observe:
* Notice that the sky inside the rainbow is brighter than the sky outside it. That's because the water droplets inside the rainbow reflect and refract sunlight straight back to you, making that part of the sky seem brighter. Raindrops above the rainbow send light away from you.
* See the base of the rainbow, where it appears to meet the ground? The rainbow is brighter there, because sunlight reflects off large and small raindrops at the base. Only small raindrops are at the top, and they reflect less light. But when people first noticed the bright ends of the rainbow, they imagined that the glimmer had another explanation. A pot of gold, perhaps?
* The order of the colors of the rainbow are always the same. Have you ever heard of Roy G. Biv? That's one way to remember the proper order: Red, Orange, Yellow, Green, Blue, Indigo, Violet. That's always the order - except when it's reversed, which is what happens in a secondary rainbow (See next item.) But whoever heard of Vib G. Yor?
* Look slightly above the rainbow. Do you see a second, faint rainbow above? It's a double rainbow! And if you do see a double rainbow, notice:
1. The dark band of sky between the primary (main) and secondary rainbows. This is called "Alexander's band." It's named after Alexander of Aphrodisias, who pointed out that Aristotle's theory of rainbows failed to take this feature into account.
2. Notice the order of the colors in the secondary rainbow. They're the reverse of the colors in the first rainbow.
The second rainbow is created when light rays are reflected twice inside the raindrop. A triple rainbow is possible, too, but very difficult to see, says meteorologist Raymond Lee. He claims to have found three reliable accounts of triple-rainbow sightings, but had to scour 150 years of history to find them. The third bow is difficult to see because it's very near the bright sun.
* Primary rainbows always appear 42 degrees above the top of the shadow cast by your head. (That's the simplest way to describe what scientists call the "antisolar point," the point directly opposite the sun.) The antisolar point is usually below the horizon. But if you get up high enough and conditions are right, you might see a rainbow as a circle, rather than as an arc. People in tall buildings, airplanes, and on mountains have seen these circular rainbows.
* Often you see only part of a rainbow. This is because the conditions that create the rainbow do not extend all the way across the sky. Where water or sunlight are absent, so is the rainbow.
* Taking a picture of a complete rainbow can be frustrating, whether it's swooping across the sky or dancing across a spray of water from a hose. Here's why: Most 35mm cameras have a field of view of 40 degrees (out of 360). A rainbow takes up more than 40 degrees of sky. And because it is an image, not an object, you can't back up to fit more of it into your picture. Use a wide-angle lens.
Source: Meteorologist Raymond Lee. His book, 'The Rainbow Bridge' (with Alistair Fraser) comes out next year.
(c) Copyright 1999. The Christian Science Publishing Society