BOSTON — Raymond Lee has been chasing rainbows since 1985 - and he still hasn't caught one.
"Trying to reach the end of a rainbow," he says, "is an introduction to the difference between an object and an image." Rainbows may look like objects, but they're just beautiful images created by rain and sun.
You've probably seen rainbows. Just after a rain, when the sun comes out, run outside and turn so your back is to the sun. See a rainbow?
You can also see rainbows in the fine spray from a garden hose or a sprinkler. Waterfalls sometimes create rainbows. Swimmers doing the breaststroke in an outdoor pool have created so much misty spray by their faces that they can see two rainbows!
It was a French scientist who figured out how rain and sun make rainbows in the 1630s.
Ren Descartes (Ruh-NAY DAY-cart) began by making a giant model of a raindrop. Raindrops, he knew, were round (not teardrop-shaped, as you see in cartoons and drawings). He used a spherical glass flask filled with water as a model. Then he began observing, measuring, and calculating.
Descartes knew that light bends when it passes from air to water, and vice versa. He also knew that when light enters a round lens - a raindrop - it will bounce off the back wall and come out the front. Scientists call this bending and bouncing "refracting" and "reflecting." This is the key to the rainbow.
Parallel rays of light enter a raindrop. As they enter the droplet, they bend. When they hit the back of the drop, they are reflected. When Descartes drew a big raindrop and mathematically traced how rays of light would travel through it, he found an interesting thing. More light came out of one particular spot than from any other. This concentration must have something to do with rainbows, Descartes decided. He was right.
But what about a rainbow's colors? Where do they come from? Descartes didn't know.
It was Sir Isaac Newton of England who figured out, 30 years later, that white light is made up of different wavelengths. We see these different wavelengths as different colors. Different colors of light bend (refract) differently. Some bend more than others do. When white light is bent by entering and exiting a drop of water, the wavelengths spread out. We see a rainbow!
The rainbow you see is your own
And not only do we see a rainbow, everyone sees his own, personal rainbow.
"Rainbows are like your shadow," says Lee, an adjunct in meteorology at the United States Naval Academy in Annapolis, Md. "They travel with you."
Here's how it works: Picture a huge cone, cut in half lengthwise, and laid 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.
A rainbow doesn't last long - only about half an hour. Then the conditions that created it change or move along. You have to move fast. "I've interrupted dinner a few times," Lee says. "You have to be in the right place at the right time," to see a rainbow.
The "right place" can be almost anywhere there's liquid water in the air and lots of sunshine. (Slanting sunlight is best; you won't see many rainbows at noon.) Places with fast-moving rainstorms are best: The tropics, in other words. Hawaii is famous for the rainbow photographs taken there at sunset.
But some researchers can't wait for a rainstorm or fly to Hawaii. So they make and study rainbows in a lab, using lasers. But to Lee, chasing rainbows outside is more fun.
A scientific view of a rainbow's colors
"I've always been interested in very impractical stuff," he says. In his studies, he's found that rainbow colors aren't as pure as popular lore has it. Rainbows are spectacular, but "the colors of the rainbow are comparable to those of some vegetation or the sky," he says. "The average paint store has a better collection of colors."
Knowing that, does he still get excited when he sees that it's raining?
"Absolutely," he says. "I grab my camera and I'm ready to fire."
(c) Copyright 1999. The Christian Science Publishing Society