How do rainbows form?

A rainbow appears when sunlight interacts with raindrops in the air. As light enters a droplet, it slows and bends (refraction); different colors bend by slightly different amounts, spreading white light into its spectrum. The light then reflects off the back of the droplet and bends again as it exits, sending the separated colors back toward you. Because each color leaves at a slightly different angle — red at about 42 degrees, violet at about 40 — you see an arc of colors, always with the Sun behind you and rain in front. A rainbow is actually a full circle, but the ground usually hides the lower half; from an airplane you can sometimes see the whole ring.