It is one of the first questions many children ask: why is the sky blue? The…

It is one of the first questions many children ask: why is the sky blue? The answer, which scientists finally worked out in the nineteenth century, has to do with the way sunlight interacts with the air.

Sunlight looks white, but it is actually a mixture of all the colours of the rainbow, from red to violet. Each colour travels as a wave of a different length. Red light has the longest waves, while blue and violet light have much shorter ones.

When sunlight enters the atmosphere, it meets countless tiny molecules of gas, mainly nitrogen and oxygen. These molecules are far smaller than the waves of visible light, and they scatter some of the light in every direction. Crucially, they scatter short waves much more strongly than long ones. As a result, blue light is spread across the whole sky, reaching our eyes from every direction, while most of the red and yellow light continues in a straight line from the sun. This effect is called Rayleigh scattering, after the British scientist Lord Rayleigh, who explained it mathematically.

If short waves are scattered most, why is the sky not violet? There are several reasons. Sunlight contains less violet than blue in the first place, and some violet light is absorbed high in the atmosphere. Most importantly, our eyes are much more sensitive to blue than to violet. The combination of colours that reaches us is therefore seen as sky blue.

The same process explains the colours of sunrise and sunset. When the sun is low on the horizon, its light has to travel through much more air before it reaches us. By the time it arrives, most of the blue light has been scattered away to the sides, leaving the reds, oranges and yellows that make evening skies so beautiful. Dust and pollution in the air can make these colours even stronger.

Clouds, on the other hand, are white because their water droplets are much larger than gas molecules. Large particles scatter all colours of light more or less equally, so the light that comes from clouds remains white or, when the clouds are thick, grey.

Interestingly, things are different on other planets. On Mars, where the thin atmosphere is full of fine reddish dust, the daytime sky has a pale orange colour, while sunsets can appear blue.