Question: Why is the sky blueish?

Ben Cropper answered on 6 Mar 2020: last edited 6 Mar 2020 12:35 pm

This is because of something called Rayleigh scattering.

What this means is that when light passes through a gas, different colours are more likely to bounce off the atoms in the gas, with longer wavelength (redder) light being less likely to bounce off and shorter wavelength (bluer) light being more likely. (You could ask why that is but the answer I’m afraid is “it’s really complicated”, if you study enough physics then you can answer your own question later!)

Anyway, the red light from the sun doesn’t bounce around the air very much and goes straight from the sun to the ground. The blue light however bounces around the sky in all directions and into your eye, so you see blue coming from all directions.

If you want to annoy your science teacher, ask them ‘why is the sky blue’, and they might say something like what I said in this message. If they do, then say ‘Then why isn’t the sky purple?’ (purple is a shorter wavelength colour than blue, it goes in the order of the rainbow). The answer to this is the much more boring ‘human eyes are worse at seeing purple than blue’ which is no good in physics lessons on the subject, so it might catch them out!