Why Is the Sky Blue?

Someone I knew used to say that the sky is blue because that is the color of light reflected off the oceans. That’s an interesting explanation but the actual reason is something called “Raleigh scattering.” The visible spectrum of light includes long wavelengths (seen as orange and red), medium wavelengths (seen as green) and short wavelengths (seen as blue). Light moves in a straight line as long as nothing disturbs it. When light passes through the Earth’s atmosphere, it keeps moving in a straight line until it bumps into a dust particle or a gas molecule. The long and medium wavelengths of light can pass straight through the atmosphere unaffected. However, the shorter wavelengths collide with gas molecules in the air. These collisions scatter the short wavelengths in different directions. We end up seeing the sky as the same color of light that was scattered. Since we see short wavelength light as blue, we see the sky as blue.