Why is the sky blue?

What a great question!

The light coming from our glorious sun is made up, in part, of all the colours of the visible spectrum – we can see this in action when we pass white light through a prism.

Each colour is defined by its wavelength. Red light has a wavelength of about 7 x 10^-7 metres and blue light has a shorter wavelength of about 4 x 10^-7 metres. So how do we use this information to understand the colours in the sky?

Light coming directly from the sun overhead looks white (although, of course, we must never look directly at the sun). The sky away from the sun itself, on a clear day, looks blue. And the sky at sunrise or sunset looks orangey red. Why do we get all these different colours?

When sunlight enters our atmosphere, it hits all the tiny particles and gases in the air. For those particles much smaller than the wavelength of light, the light is absorbed and re-emitted in all directions - a process called Rayleigh scattering. But not all light scatters equally. Because blue light has the smallest wavelength and the highest energy, it is scattered much more, in fact ten times more, than red light, so we see blue skies.  

However, light from the sun directly overhead looks white. This is because the light is passing through much less atmosphere so only a very small amount of blue light has had a chance to be scattered. Conversely, on the horizon, during sunsets and sunrises, light has to pass through much more atmosphere before it reaches our eyes, so all the blue gets scattered away and the red passes through almost unimpeded. 

Finally, why are clouds white on a sunny day? This is because clouds are made up of water droplets which are relatively huge compared to the wavelength of light, and in this case all colours are scattered equally, irrespective of their wavelength, and so the light remains white.

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