Question: Why is the sky blue?

Andrew Yool answered on 29 Jun 2020:

Hi Finley.
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This is caused by a process we call Rayleigh scattering. Light is made up of lots of particles called photons that cover a wide range of wavelengths [*]. We perceive these photons with different wavelengths as being different colours. Shorter wavelengths are blue, longer wavelengths are red, with green in the middle. The sun’s light is a mix of all the colours of the rainbow, and when we see these all together, we see the light as being white. However, when light reaches the Earth’s atmosphere, photons with different colours behave differently. The shorter wavelength blue ones are scattered when they interact with molecules in air. That means they keep changing direction, so that instead of looking like they’ve come from the sun (which they have), they instead look like they’re coming from all directions. The other photons with longer wavelengths aren’t affected in the same way, and the net result is that when we look up at the sky it looks blue because we’re seeing these blue photons bouncing all over the place.
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When the sun is very low in the sky, it passes through more atmosphere, scattering away much of the blue and green light. This gives the low sun the orange-red colour we associate with sunrise and sunset. This scattering of light is also why, even when the sun has set below the horizon, that the sky can still be lit up.
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Finally, I should add that blue skies are not a universal feature. The absence of an atmosphere on the Moon means that its sky is black – there’s no scattering. Meanwhile, dust in the Martian atmosphere makes its sky appear red.
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Anyway, I hope that this helps!
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[*] There’s a whole other set of questions about what we mean by particles, wavelengths and light waves when we talk about electromagnetic radiation. The above answer is quite a simplication.