Why is the sky blue?

Our patrons are a curious bunch. Or maybe they just want to test us out. :-) Here’s the reason the sky is blue. This explanation was written for us by Carl Aude, who works at NASA. The answer is based in part on the explanation in
Hecht, Eugene. Optics. 2nd Edition. New York: Addison-Wesley, 1988. pp. 293-294.

Light from the sun, like light from many sources, contains all frequencies (colors) of visible light. All the frequencies together look white, and are called white light. The first thing you might learn about light is it goes in a straight line. Next you learn there are four exceptions to this rule: reflection, refraction, diffraction, and scattering. Scattering is when light bounces off things like molecules.

If you were in space the sun would look very bright and the sky would look absolutely black, since light normally goes in a straight line. But here on Earth, light is scattered by molecules of air as it passes through the atmosphere. Lord Rayleigh was the first to research the dependence of scattering on the frequency of light. He found that scattering increases in proportion to the fourth power of the frequency of the light. This means that light of higher frequencies (blue) is scattered much much more than light of lower frequencies (red). While most light passes straight on through to the Earth’s surface, there is so much sunlight to start with that significant amounts of light are scattered and due to its higher frequency, the bulk of the scattered light is blue. It is scattered in all directions, but much of it makes its way to the Earth again, coming in from all directions, and this makes the sky blue. This is also the reason for the color of sunsets. As the sun approaches the horizon it comes to us through a greater angle and passes through the atmosphere for a much greater distance. First, this causes the longer wavelengths of red, orange, and yellow to be scattered more so that they color the nearby sky. Second, it causes the blue light to be scattered away even more and not reach us as much.

You can do an experiment to learn more about this process. The experiment is described at http://www.exploratorium.edu/snacks/blue_sky/.