1Few everyday spectacles are as universally admired as a brilliant red sunset sinking slowly below the horizon at the end of a clear day.
2Yet the explanation for this glorious display lies in the surprisingly simple physics of how light behaves inside our atmosphere.
3Sunlight looks perfectly white to us, but in fact it contains every colour of the rainbow mixed together in equal measure.
4Each of these colours travels through space as a wave, and every colour has a slightly different wavelength from the others.
5As sunlight enters the atmosphere, it collides with countless billions of tiny molecules of nitrogen and oxygen floating in the air.
6Blue light, because of its short wavelength, is knocked off its path and scattered in every direction far more easily than red light.
7That scattered blue light reaches our eyes from every part of the sky, which is precisely why a clear daytime sky appears blue.
8At sunset, however, the sun sits low on the horizon, so its light must travel through a much thicker layer of air.
9During this longer journey, nearly all of the blue light has been scattered away before the sunlight can finally reach our eyes.
10Only the longer red and orange waves survive the trip, and they are the colours that dominate the evening sky around us.
11Dust, smoke, and pollution scatter even more light, which is why sunsets after volcanic eruptions can remain astonishingly vivid for months.
12So the next time you watch the sun go down, remember that you are seeing invisible physics painted across the whole sky.