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What causes a rainbow?

A rainbow looks like an arc as you can’t see the bottom half because of the ground. In the air, it might look like a disk.

iStockphoto/Thinkstock

There's something about this atmospheric phenomenon that has inspired awe in people since ancient times. In the Book of Genesis, God put a rainbow in the sky after the Great Flood and told Noah it was a sign of "a token of the covenant between me and Earth" [source: Biblos]. The ancient Greeks went further, and decided that the rainbow actually was a goddess, whom they named Iris. But they made her an ominous figure -- the bearer of the Olympian gods' tidings about war and retribution [source: Lee and Fraser, pg viii]. And over the centuries, great minds ranging from Aristotle to Rene Descartes sought to figure out what process created rainbows' striking array of colors [source: Broughton and Carriero].

Since then, though, scientists have nailed it pretty well. Basically, rainbows are caused by the droplets of water that remain suspended in the atmosphere after a rainstorm. The droplets have a different density than the surrounding air, so as sunlight hits them, the droplets act as tiny prisms, bending the light to break it up into its component wavelengths, and then reflecting them back at us. That in turns creates the arc with bands of colors of the visible spectrum that we see. Because the droplets have to reflect the light at us, in order to see a rainbow, we have to be standing with our backs to the sun. We also need to be looking up from the ground at an angle of approximately 40 degrees, which is the rainbow's angle of deviation -- i.e., the angle at which it bends sunlight. Interestingly, if you're in an airplane and you see a rainbow from above, it actually may look like a disk, rather than an arc [source: Physics Classroom].

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