Here's a question children ask, and adults usually answer half-correctly: why is the sky blue?

The standard answer is right as far as it goes. Sunlight contains all the colours of the rainbow. As it passes through the atmosphere, it bumps into gas molecules, mostly nitrogen and oxygen. These molecules scatter short wavelengths of light — the blue end of the spectrum — much more strongly than long wavelengths like red. This is called Rayleigh scattering, after the physicist Lord Rayleigh.

So blue light gets bounced all over the sky, and wherever you look, some of it is coming toward you.

The catch

Violet light has an even shorter wavelength than blue. That means it's scattered even more.

So shouldn't the sky be violet?

Reason one: there's less violet to begin with

Sunlight isn't an even mix of colours. It contains less violet than blue, and some violet is absorbed high in the atmosphere.

Reason two: your eyes

Our eyes detect colour using three types of cone cells, roughly sensitive to red, green and blue light. They're much less sensitive to violet. And the scattered skylight that reaches us is a mix of wavelengths, not pure violet or pure blue. Our brains combine the signals from all three cone types, and the result is what we perceive as sky blue — a slightly whitish blue.