Question: why is the sky blue?

Rick Smith answered on 26 Jun 2013:

This is a really good question, the exact reason uses some pretty complicated physics but hopefully I can explain the idea well enough without writing too much. Sun light is white, made up of all the colours mixed together. Since light is made up of waves, the different colours are waves with different wavelengths (the space between tops of the waves). When sunlight passes through the sky, the molecules in the air are just the right size to scatter some of the light, so the light bounces off the air molecules and instead of moving in a straight line the light bounces off in random directions. Some of this comes down towards us on the ground and when we look up at the sky we see this light that has been scattered.

It just so happens that the air molecules and the wavelength (colour) of blue light match up perfectly to only scatter the blue light. The other colours pass the air molecules in a straight line so we don’t see them when we look up.

So the sky is blue just because of the coincidence that the air molecules are just the right size to scatter blue light. If air molecules were a different size then the sky would scatter another colour, so the sky could be red or green. If the size of air molecules didn’t match any of the colours of light then the sky wouldn’t scatter any light down to us, and the sky would always be black!