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Science in the Sky: The Reasons for Rainbows

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There it was, arching over the sky in front of me, a gash of color through a gray, rainy afternoon. I had been walking home from a job I hated, depressed about my prospects for the future, and the rainbow was exactly the mood lifter I needed. I think it’s technically impossible to be sad before such spontaneous color and beauty. There’s no wonder that the rainbow has been a symbol for a number of revolutionary movements in history, from the German Peasant’s War in the 16th century to the establishment of the LGBT community in the 1970s. It’s a sign of hope for a new day.

Every culture seems to have its own myth about how rainbows are formed. The ancient Greeks considered them paths between Earth and Heaven made by the messenger goddess, Iris. The Chinese, Hindus, and Celts all had their own lore about rainbows as well. The phenomena are mentioned in the Judeo-Christian Bible and the Epic of Gilgamesh. But what is the scientific explanation for them?

Despite rainbows’ miraculous appearance, the science behind them is actually basic optics, the principles of which were discovered by Isaac Newton in the 17th century and developed by Thomas Young in the early 19th century.

The Bending of Light
Rainbows are formed whenever sunlight shining from behind an observer on the ground is refracted through water droplets in the air. Refraction is considered the “bending” of light, but can be more accurately described as the process of light changing speed as it moves through different densities.

Imagine that you’re swimming through a pool of water. Now imagine that water turning into mineral oil. You’ll be swimming slower, since the thickness of the oil will be more difficult to move through. And when that mineral oil turns to strawberry Jell-O, you’ll be moving even slower.

Light does exactly the same thing as it moves through various mediums. It must change its frequency, or speed, depending on whether it is traveling through air, water, or glass. Different colors have different frequencies, which is why a beam of white light separates into its component colors as it moves through a prism. In the sky, water droplets after a rainfall act as a prism for sunlight, breaking it into the component colors of red, orange, yellow, green, blue, indigo, and violet.

There are as many mnemonic devices to remember the order of these colors as there are people who love rainbows. You may have learned one of the following in elementary school: ROY G. BIV, Richard of York Gave Battle in Vain, or (my favorite) Rainbows Over Your Grass Bring Instant Victory.

The Wide World of Rainbows
Just as light has many variations in appearance, so do rainbows.

They aren’t objective phenomena. In other words, because the perception of refracted light depends on the angle of the observer, it is impossible to pinpoint exactly where a rainbow appears. And the rainbow itself is only the beginning. There are double rainbows that result from light reflecting within the raindrop and exiting at a different angle, so that it appears higher than the original arc. Because a double rainbow is really a reflection of the first, you’ll notice that it appears fainter and with reversed color order. Triple and quadruple rainbows exist, but are very rare.

Rainbows also vary in size and shape depending on the density of the water droplets. Because seawater has a higher density than rain water—this is why it’s easier to float in the ocean—and a higher refractive index, the rainbows that appear in sea spray are smaller than “true” rainbows that appear after a storm. Rainbows that are reflected below the horizon over a body of water are simply called “reflected rainbows.”

Supernumerary rainbows are several faint rainbows on the inner side of the main rainbow. They are slighting detached and their color bands do not fit the usual pattern. Their existence was the first indication of light’s wave nature rather than the particle theory adopted after Newton.

Unweaving the Rainbow
Keats lamented the scientific deconstruction of the rainbow, but even understanding the science behind it does not take away from the beauty and wonder of this natural occurrence. I like the idea that a rainbow is personal; no two people can view it exactly the same way at the same time. Someone standing next to you will also see a rainbow, but the colors and intensity will differ slightly. That rainbow I saw walking home from work, as well as the future it encouraged me to pursue, are all mine.

Updated April 16, 2010


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