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.