Why is the sky blue?
(Lansing State Journal, March 20, 1996)
If you look at sunlight through a prism, you will find that "white" sunlight is actually composed of a rainbow of colors.
Light can be described as a wave, and different colors of light correspond to different wavelengths. For example, red light has a long wavelength compared to blue light. As sunlight passes through the atmosphere, the atmosphere selectively filters blue light using a process called scattering. An example of scattering can be seen in a smoke-filled room. When light enters the room, the smoke particles scatter the light, resulting in shafts of visible light. The smoke particles, however, have little effect on the color of the scattered light.
Selective scattering (preferential scattering of one color of light over other colors) occurs when light scatters off particles that are much smaller than the wavelengths of the colors. In the case of our primarily nitrogen filled atmosphere, the shorter wavelengths (blue) are scattered much more strongly than the longer wavelengths (red). Thus, as sunlight passes through our atmosphere, blue light is preferentially scattered, and becomes visible to the eye.