What causes rainbows?

Rainbows are caused by the refraction, reflection, and dispersion of light in water droplets. When sunlight enters a droplet of water, it slows down and bends due to the change in medium, splitting into its constituent colours (a spectrum). The light is then reflected off the inside surface of the droplet and refracted again as it exits, creating the arc of colours. The seven main colours of a rainbow are red, orange, yellow, green, blue, indigo, and violet. Rainbows are always seen opposite the sun, and the observer’s position relative to the droplets determines visibility. Double rainbows can occur when light is reflected twice inside the droplets.