Why the sky is blue

The weather has connections with many areas of fundamental science, some far from obvious.

Albert Einstein’s fame as the father of relativity theory rather obscures the fact that he was something of a polymath. He also contributed, of course, to that other great pillar of twentieth century physics, the quantum theory. But did you know that his PhD thesis actually explains rather well what happens in a cup of sweet tea? Or that it was Einstein, in 1910, who proved once and for all why the sky is blue?

We are so used to the fact that blue light comes to us from all over the sky that few people stop to wonder where the light comes from. If our light comes from the Sun, and light travels in straight lines, why isn’t the bright Sun set in a black sky?

The answer is that sunlight has got scattered by the atmosphere of the Earth, bounced around until it comes at us from all directions. But sunlight doesn’t get scattered evenly.

The first steps towards explaining the puzzle were taken by John Tyndall, an Irish physicist who succeeded Michael Faraday as Superintendent of the Royal Institution in 1867. Two years later, he discovered the “Tyndall effect”, by which large particles in solution scatter a beam of light shone into the liquid.

Tyndall — who was also one of the first people to investigate the carbon dioxide greenhouse effect — guessed that something similar happens in the atmosphere, with sunlight being scattered by dust particles suspended in the air. Sunlight itself contains all the colours of the rainbow, but experiments showed that shorter wavelength (blue) light is scattered more easily than longer wavelength (red) light.

In fact, the dust scattering invoked by Tyndall nicely explains why sunsets are red. But it does not explain why the sky is blue. To do that much scattering requires a very large number of very small particles — atoms and molecules. It was Einstein who put the right numbers in to the calculation, explaining why the sky is blue and providing proof that air really is made of molecules, something that still needed proof, in the eyes of some scientists, at the end of the first decade of the twentieth century.

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