You probably all know how rainbows form, if not from being told as a child, then from having to explain it to a child as an adult. But here's the basic version anyway in case you, uh, forgot. The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.

Light in a vacuum – as far as our best theories and experiments suggest – zips along at a constant speed C. But as it enters or leaves a medium, such as air or water, it slows down and changes direction, known as refraction. This is why it can sometimes look like people's legs / heads have been ripped clean off in a pool.

Sunlight contains many different wavelengths of light, which are slowed by different amounts as the sunlight enters a medium. Violet – the shortest wavelength in the visible spec

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