Why do Rainbows Form?

Rainbows are a striking and beautiful meteorological phenomenon caused by the refraction of light. In order for rainbows to form, the sun must be near the horizon, and there must be a heavy fog, mist, or rain opposite the sun. An observer standing between the sun and the rainbow would see a 180 degree arc of colors which run through the visible spectrum from red to violet. In some cases, if conditions are right, double rainbows will form, with a pale rainbow in reversed colors above the primary rainbow, and in other instances, a fully circular rainbow has been observed, usually from inside an aircraft flying over the Earth.

Although rainbows have been observed, written about, and depicted in art for centuries, their cause was not understood until physics began to explore the properties of light. Essentially, the cause of a rainbow is the reflection of sunlight through individual drops of water. Light enters the water and reflects from the opposite side of the water drop, bouncing back through the point of entry. Because the angle or refraction of the light changes as it bounces back, it filters the light into different colors, which arrange themselves with red at the top and violet at the bottom because red is the longest wave length, and violet is the shortest.

When double rainbows form, the light refracts twice, causing a fainter rainbow to mirror above the primary one. Potentially, a triple rainbow could form as well, although it is extraordinarily rare. As soon as the water dissipates enough or the sun moves, the rainbow will disappear, because conditions are no longer optimal for the refraction of the light through the drops. It is also possible for a moonbow to form, although because humans have difficulty distinguishing color in the dark, it usually appears in shades of white and gray.

People seeking rainbows should go outside when the sun is near the horizon and there has been a heavy rain, or there is an active storm. If the observer turns his or her back to the sun and looks in the direction which is immediately opposite the sun, he or she may see rainbows if the conditions are right, with the apex of the arc located directly opposite the sun. In some instances, light conditions are right for rainbows to form in close proximity to an observer: otherwise, the rainbow will continue to look as though it is moving away from the pursuer until it finally disappears altogether, because the observer has actually passed through it.