Rayleigh scattering
n.
The scattering of electromagnetic radiation by particles with dimensions much smaller than the wavelength of the radiation, resulting in angular separation of colors and responsible for the reddish color of sunset and the blue of the sky.
hy is the sky blue: Discussion
Transmitted light (from the sun, light bulbs, fire, etc) is made up of a spectrum of colors. The longest wavelengths of light are on the red end of the spectrum and the shortest wavelengths are on the blue/violet end of the spectrum.
When transmitted light such as sunlight enters our atmosphere it collides with the oxygen and nitrogen atoms. The color with the shorter wavelength is scattered more by this collision. Because violet and blue are the shortest wavelengths the sky appears to be violet / blue. But because our eyes are more sensitive to blue light than they are violet light, we perceive the sky as blue.
Our eyes contain thousand of rods and cones, which are the receptors for light. Whenever one of the 3 Stooges pokes you in the eye you see a giant blue spot. This is because the blue receptors have been activated. Blue is one of the primary colors and thus more easily activated and seen by our eyes.
Blue is also how I feel when my baby leaves and my hound dog dies. Also, how I feel when the cops pull me over and I see their blue lights flashing in my rear view mirror. Then, again, blue is the color of the K-mart special, so this color isn't all bad.
Why is the sky blue: Summary
So, why is the sky blue? It is because blue light from the sun strikes the air molecules and scatters and our eyes perceive it as blue.
Why is the sky blue: Short Summary
Why is the sky blue, you ask? Blue in sunlight collides with air molecules and our eyes see it as blue.
Why is the sky blue: Condensed
Sunlight collides with air, scatters blue wavelengths.
Why is the sky blue: Ultra-Condensed.
You are seeing things. Stop asking.