What causes rainbows to form?

Rainbows form when sunlight passes through water droplets in the atmosphere, creating a spectrum of colours. As light enters the droplet, it slows down and bends (refracts), then reflects off the inside surface of the droplet and exits while bending again. This separation of light into its component colours produces the familiar red, orange, yellow, green, blue, indigo, and violet arc. The angle at which light refracts and reflects determines the visibility of the rainbow, with the most vivid displays occurring when the Sun is low in the sky.