Rainbows

How are the colours in a rainbow formed?

The ideal conditions for a rainbow occur when a brief morning or evening shower is followed by a rapidly brightening sky. Large numbers of water droplets are still present in the air, and the Sun shines onto them from a relatively low position in the sky.

This is the reason why morning rainbows only appear in the west and evening rainbows only in the east. Rainbows are only ever seen on the side of the sky opposite to the Sun.

The fascinating technicolour display that is a rainbow is the result of white sunlight being broken down by refraction into its component colours. This happens on the curved surfaces of the almost spherical droplets of rain, which is also why rainbows have their characteristic arched shape.

As is the case with white light broken down by a prism, blue light, with its shorter wavelength, is refracted at a greater angle than red light, which has a longer wavelength. This is why the colours appear in their familiar sequence – red, orange, yellow, green, blue, indigo and violet.

Sometimes it is possible to see another, weaker, secondary rainbow. This is created by the light waves being refracted a second time in the water droplets, but, with the secondary rainbow, the colours appear in reverse order to those of the primary rainbow.

Rainbows