I have to admit that I wasn’t one of those kids, the kids who asked questions that made their parents proud, but also feel a bit stupid for not being able to provide the answer. It never really occurred to me that there was a reason that the sky is blue. I mean, it’s just blue!
And that’s the beauty of Physics, it just works, and you don’t have to know how or why, or even that there is something going on. But then when you do know, it’s even more amazing. I personally can’t imagine the sky being any other colour than blue (well, apart from grey and drizzly, but that’s the cloud’s fault), but it might have been different.
Do you remember the posts on how rainbows are formed? It was to do with the refraction of sunlight through the rain droplets, which works in the same way as a triangular prism does to scatter the white light into its constituent parts.
All the rainbow colours separate because their wavelenths are all different, so when they change direction (refract) through the water (or triangular prism), the different colours take different paths. Thus they come out the other side separated out to the colours we all know. Blue happens to be at the short wavelength end of the spectrum, whilst the longer wavelengths have the red colours.
It’s almost the same property that creates the blue sky, but this time rather than refracting through water molecules, the process happens higher up in the atmosphere. At this level it’s the air molecules that affect the light. When the incoming sunlight hits the atoms, it scatters, an effect which is called Rayleigh Scattering (obviously discovered by Lord Rayleigh, in the grand tradition of naming things after yourself). This sends the various parts of the white sunlight in a lot of different directions.
We see the blue because the short wavelength light (blue end of the spectrum) tends to scatter at large angles, which is what is required for us to see the lights after it enters the atmosphere far above the earth’s surface.
And whilst we’re on the subject, why is the sunset red? Well as the sun sets, the angle changes between us, the sun and the atmosphere. So now we see the red light that is scattered at shallower angles.