THE RIGHT LIGHT
When it stops raining and the sun comes out, stand outside with your back to the sun. If conditions are just right, you’ll see a rainbow. (But hurry—it will 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 water droplets inside the rainbow reflect and refract sunlight straight back to you. 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. This explains the legend of the “pot o’ gold”: When people saw the bright ends of a rainbow, they imagined that the glimmer had a magical explanation.
The order of the colors of the rainbow is always the same. Have you ever heard of Roy G . Biv? That’s the standard mnemonic device for remembering 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. For that, you need to remember Vib G. Yor. Some details:
• Look slightly above the rainbow. Do you see a second, faint rainbow above? It’s a double rainbow—and if you do see it, 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 Aphrodislas.
2. Notice the order of the colors in the secondary rainbow. They’re the reverse of the colors in the first rainbow. The second raindrop. A triple rainbow is possible, too, but very difficult to see. Meteorologist Raymond Lee claims to have found three reliable accounts of triple-rainbow sightings but had to scour 150 years of history to find them. The third rainbow is difficult to see because it’s very near the bright sun.
THE WHOLE THING?
• 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 circular rainbows.
• Often you see only part of a rainbow. This is because the conditions that create the rainbow don’t extend all the way across the sky. Where water or sunlight is 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’s an image, not an object, you can’t back up to fit more into your picture. Use a wide-angle lens.
THE RAINBOW YOU SEE IS YOUR OWN
There isn’t one rainbow—there’s one for each viewer. Everyone sees his own, personal rainbow. Here’s how it works: Picture a huge cone. It’s on the ground, flat side down. Picture yourself standing at the point of the cone. The raindrops that are bending and reflecting the sunlight that reaches your eye as a rainbow are located on the surface of that cone. But someone standing right next to you is seeing a rainbow generated by a completely different set of raindrops along the surface of a different imaginary cone. So next time you see a rainbow, remember—it’s not in the sky, it’s in your eyes… and you’re the only one who can see it.