Question: What causes a rainbow?

Iain Bethune answered on 23 Jun 2015:

A rainbow is caused by a physical effect called ‘refraction’. Light from the sun is made up of lots of different wavelengths of light, mixed together they appear yellow-white. When light travels from one ‘medium’ to another e.g. from air into water, then the light bends, and the amount of bend depends on the wavelength of the light. You can see this if you put a straw or spoon into a glass of water, then look from the side and you will see it appear to bend where it passes through the surface of the water.

When sunlight passes through a rain-drop, then the different wavelength refract by different amounts, splitting them up into individual colours – each colour is produced by a different wavelength. The shape and location of the rainbow are because for you to be able to see this, the sun, the rain droplets and your eyes need to be lined up in exactly the right fashion. So you can never get to the end of the rainbow and find a pot of gold – sorry!