Why is the sky blue?

A view of clouds and sky5Seriously, why is it blue, not green or orange? This is one of the classic science questions. Simple enough that a five-year old can ask it, but containing a lot of science – the answer tells us a lot about how the universe works. I was reminded of it on Monday. I was at a biological physics one-day meeting, and a guy called Tim Hunt was giving a talk. It was an entertaining and interesting talk, I really enjoyed him sharing not only interesting science but his child-like delight in discovering stuff out. Not bad for a 70-year old, but Prof Sir Tim Hunt FRS (2001 Nobel Laureate in Physiology or Medicine) shows you can get every award going, and still be a big kid asking the simple questions. 

Anyway, back to why the sky is blue. It is blue because the atmosphere scatters short wavelength, high frequency, blue light more than longer wavelength, lower frequency, red light. Here scatters is a technical term meaning change the direction light moves in. When you look up on a summer’s day and see blue sky, you are seeing predominantly blue light that has come from the Sun, hit the atmosphere in front of you, and richocheted off at an angle into your eyes.

OK, so the atmosphere looks blue as blue light from the Sun is scattered into your eyes by the atmosphere. But why is blue light scattered more than red? As Tim Hunt said, quoting Jean Perrin, it is because the atmosphere is made of, tiny, molecules.

The full formula for light scattering off things is pretty unattractive – it needs a fair of amount of fun calculus with Maxwell’s equations (the differential equations that light and all other forms of electromagnetism) to get it.  But the point is that if the light is scattered by objects much smaller than the wavelength of light, then the amount scattered varies as the frequency f4, i.e., it increases as the 4th power of frequency f . Then the scattering is much higher for high frequency blue light, than for red light. But if the light is scattered from objects as large or larger than the wavelength of the light, the frequency dependence of the scattering is much weaker. Blue and red light are scattered almost as much – which would give a white sky.

So, if the scattering varies a lot between blue and red light – and if we ever get a sufficiently high ratio of sunshine to cloud we will see that it clearly is – then this is evidence that the atmosphere has tiny things in it: atoms.