Question: how does a rainbow work?

Edward Banks answered on 16 Mar 2020:

The short version is that water droplets in the air act as a prism and split the light out into individual parts.

Light that comes from our sun is composed of many many different colours of light, all travelling together- our Sun emits all colours of visible light, with a peak right in the green area of the spectrum. Now when this light hits water droplets in the air, it gets refracted; essentially it changes direction a little bit.
A thought experiment to explain how this works: imagine you are pushing a shopping trolley along a paved car park. You’re moving towards the edge of the paving, the other side of which is muddy. If you are going at a bit of an angle rather than head on, one of you wheels will hit the muddy ground first. If it is your left wheel, suddenly there will be a lot more friction on the left side and your trolley will veer off to the left, even though you’re just trying to push it forwards.
So the same sort of thing happens with light; one part of it hits the higher ‘resistance’ area first (the water droplets), so it gets bent away from the path of the straight line it was travelling.
Now light has a different wavelength at different colours; and different wavelengths will mean that the light gets slowed down more or less. Blue light gets bent (more properly refracted) more than red, and the spectrum you see in-between blue and red is a result of all the different colours refracting at slightly different rates.