Question: Why is the sky blue?

Sam Geen answered on 14 Jun 2013:

Scattering! OK, so that’s a little quick, so let’s take this step by step:
1) Light comes in different wavelengths – blue light has short waves, and red light has long waves.
2) As light passes through the atmosphere, it “scatters”. This means that it has a small chance to hit an electron in one of the atoms in the air. The electron traps the light wave for a short time, and then lets it go again (this is called “Rayleigh Scattering”).
3) Light with shorter wavelengths has more of a chance to do this. This is because some particles in the air are smaller than the wavelength of red light, and so it passes right through them, a bit like a runner with long legs jumping over a hurdle and a short runner hitting it (this is probably not exactly what happens, but it’s a way of thinkin about it).
4) Therefore in the atmosphere you have a lot of blue light bouncing around, and not a lot of red light, which passes mostly in one direction from the Sun. This is (very basically!) why the sky looks blue – blue light scatters off the atmosphere better than red light.

Finally, the evening sky looks red because as the Sun moves towards the horizon, the light from it has to travel further through the atmosphere to get to us – when it’s midday, it just goes straight down to us. This means that all the blue light is scattered away, and only a dim red glow remains.

Oh, and different planets have different coloured atmospheres because they’re made of different things – Mars’s atmosphere is much thinner than Earth’s and is made of carbon dioxide, which scatters light much less. But it also has red dust in the air when storms pull up dust from the planet’s surface, which gives the sky a lovely pink colour.

Hope this helps!