Science Puzzle
Why Is the Sky Blue but the Sunset Red?
Sunlight is white: a mixture of every colour of the rainbow. Yet on a clear day the whole sky glows blue, and in the evening the setting Sun and the sky around it turn orange and red.
Why is the daytime sky blue, and why does the sunset turn red?
The Answer
Air scatters blue light far more strongly than red light. At midday that scattered blue fills the sky; at sunset the light's long journey through the air has scattered most of the blue away, leaving orange and red.
The molecules of nitrogen and oxygen in the air are far smaller than the wavelength of light. Tiny particles like these knock light off course, scattering it in all directions, and they do it much more to short wavelengths than long ones. Blue light, with a short wavelength, is scattered about five or six times more than red. This is called Rayleigh scattering, after the British physicist who worked it out in the 1870s.
At midday, sunlight comes down through a fairly thin layer of air. Some of the blue is scattered sideways all over the sky, so whichever way you look you see blue light coming at you: a blue sky. The Sun itself still looks white or pale yellow.
At sunset, the light reaching you skims through many times more air. By the time it arrives, most of its blue has been scattered out along the way, so the light that gets through is mostly orange and red. Dust and smoke in the air can make sunsets even redder. Violet light is scattered even more than blue, but the Sun sends out less of it and our eyes are less sensitive to it, so the sky looks blue rather than purple.
The principle: Rayleigh scattering. Air molecules scatter short-wavelength blue light much more than long-wavelength red, so the sky glows blue and light that has crossed a long stretch of air arrives reddened.