Question: what causes a rainbow

Sam Geen answered on 20 Jun 2013:

Unicorns!

No, sorry, it’s leprechauns.

No, no, OK. A rainbow is caused by light from the sun being split up into different colours. If you shine light through a prism, it splits up the light into the different colours (ask your parents if they have this album; the cover is exactly that: https://en.wikipedia.org/wiki/The_Dark_Side_of_the_Moon)

The reason for this is because of something called “refraction”. Waves travel at different speeds when moving through things like glass or air (which is why you sometimes hear the phrase “speed of light in a vaccuum” – light travelling through stuff is actually a bit slower. If light hits the prism at an angle, it gets bent in an angle as it changes speed. The angle depends on the wavelength of the light – red has a larger wavelength than blue, and is “refracted” (bent) less. As a result the light splits up because of the different angles they are bent at, making a rainbow.

So why do they happen in the sky? After it’s rained, the Sun comes out but sometimes there are still water droplets in the air, or it’s still raining somewhere else. The water droplets behave like prisms, splitting up the light as it passes through. This is what makes the rainbow colours.

Rainbows are in fact circles – you normally only see half of them because they hit the ground. But if you see a rainbow from a plane, sometimes you can see the whole circle – like this: http://www.dgrin.com/showthread.php?t=113283. So don’t try looking for pots of gold at the end of the rainbow – they have no end!