Why the sky is blue, and why sunsets are red

In the seventeenth century Isaac Newton passed a beam of sunlight through a glass prism and showed that it spreads into a band of colours, from red through yellow and green to blue and violet. White light is a mixture of all of them. Each colour is a different wavelength of light, from about 700 nanometres for red to about 400 for violet.

The air is made mostly of nitrogen and oxygen molecules, which are much smaller than the wavelength of light. When light meets a particle that small, it is scattered in all directions, and the physicist Lord Rayleigh showed in the 1870s that the amount of scattering is inversely proportional to the fourth power of the wavelength. In plain terms, short waves are scattered much more than long ones.

Blue light, at about 450 nanometres, is scattered roughly six times more strongly than red light at 700. As sunlight passes through the atmosphere, the blue is knocked sideways again and again, so that it arrives at your eye from every part of the sky. The red, scattered much less, mostly carries on in a straight line.

Scattering is proportional to 1 / wavelength^4
(700 / 450)^4 = about 5.9
So blue (450 nm) is scattered about 6 times more than red (700 nm).

Violet light has an even shorter wavelength and is scattered even more. The sky is not violet mainly for two reasons: the Sun emits less violet than blue, and human eyes are much less sensitive to violet than to blue. The mixture of everything that arrives looks blue to us. A different eye, in a different animal, may see the sky differently.

When the Sun is low, its light has to cross a far thicker layer of air to reach you. Along that long path the blue and green are scattered out of the beam, and what remains is orange and red. That is why the Sun itself looks red at the horizon while the sky overhead is still blue.

Dust, smoke and haze can deepen sunsets, which is why the sky after a dust storm or a volcanic eruption can be remarkable. A very clean sky tends to give pale sunsets, and a polluted one can give deep red ones, though at a cost to air quality.

Water droplets in clouds are thousands of times larger than air molecules, and they scatter all wavelengths in a similar way, a process called Mie scattering. All colours leave the cloud together, so it looks white. A thick cloud absorbs and scatters much of the light that enters it, so its base looks grey, and a rain cloud darker still.

The Moon has no atmosphere, so there is nothing to scatter light and the sky is black even at midday. On Mars the thin atmosphere is full of fine dust, which makes the daytime sky a butterscotch colour, and the area around the setting Sun appears blue, the reverse of what we see on Earth.

You can see the effect at home. Stir a few drops of milk into a glass of water and shine a torch through it. Seen from the side, the water looks faintly bluish, because the milk particles scatter blue most. Looking at the torch through the glass, the beam looks yellowish or orange, as the blue has been scattered away from it. It is a small version of a sunset.

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