Rainbows and spectra

Most of the light leaves the raindrop at an angle of about 42° from the entering sunlight.

The rainbow

How is it created?

We see a rainbow when the sun is behind us and raindrops ahead. Sunlight is refracted or bent when it passes into a raindrop and is reflected back. All electromagnetic wavelengths in sunlight are refracted to different extents and separate out as spectral colours. Red is refracted the least and violet the most.

A secondary rainbow may appear outside the original bow when the sunlight is very bright. It is fainter and its colours are in reverse order. The secondary bow is caused by two reflections of light within the raindrop, emerging at an angle of about 51°.

In theory the rainbow is part of a circle, but we only see the part which is above our horizon. Photographs of complete rain circles can be taken from space.