How Rainbows Form

St. Patrick’s Day is a holiday often associated with images of rainbows promising a path to a leprechaun’s pot of gold. For most people, however, just spotting a rainbow is enough to brighten a day.

These amazing displays of nature form when raindrops, which act like prisms, scatter sunlight. To see one, an observer must be facing a moisture source like rain or mist with the sun at their back. The sun also needs to be at a low angle in the sky, less than 42° above the horizon. The lower the sun angle, the more of a rainbow’s arc will be visible.

Refraction and reflection inside a raindrop. Credit: Met Office

Passing from the air into a denser raindrop, the light slows and refracts. Since the different wavelengths of light bend by different amounts, the white light is dispersed into the colors of the visible spectrum: red, orange, yellow, green, blue, indigo, and violet. Red, which has a long wavelength, is refracted the least and is always on the top of a single rainbow. Violet, with a shorter wavelength, is refracted the most and is always on the bottom.

The light also needs to reflect off the back wall of the raindrop towards the viewer at the critical angle of 48° before it refracts again when it re-enters the air. A lesser angle will let the light pass through the raindrop and a larger angle will allow the light to reflect straight back out of the drop.

A double rainbow is seen when the light reflects twice inside the raindrop. Since each reflection weakens the intensity of the light, the second bow appears dimmer. The order of the colors is also reversed, with blue on top and red on the bottom.

Rainbow and faint second rainbow form after a rainstorm in Bermuda. Credit: Melissa Fleming