How does a rainbow form?

A rainbow is created by the interaction of sunlight and water droplets in the air. When sunlight enters a raindrop, it slows and bends, a process called refraction, then reflects off the inside back of the drop, and bends again as it exits. Because different colours of light bend by slightly different amounts, the white sunlight is separated into its spectrum of colours, from red on the outside of the arc to violet on the inside. For you to see a rainbow, the Sun must be behind you and the rain in front, with the sunlight hitting the drops at just the right angle, about 42 degrees. This is why rainbows appear as arcs centred on the point directly opposite the Sun. Sometimes a fainter secondary rainbow appears outside the main one, with its colours reversed, caused by light reflecting twice inside the drops.