Why Is the Sky Blue?

Believe it or not, we've actually discussed this before. When we've talked about reflection nebulae, we mentioned how the blue light is reflected towards our line of sight. In the same way, the sky is blue.

Remember, that light is composed of all colors of light. When light from the Sun enters the atmosphere, the dust in the sky easily reflects the blue wavelengths because they are shorter. The longer wavelengths (red, orange, yellow) pass right by the dust particles. The blue light is dispersed over the whole sky and when we look at the sky, we see blue. Of course, when we look at the Sun (which you should never do without proper eye protection), the Sun is yellowish (more yellow light in the Sun's spectrum than orange or red.

However, at sunrise or sunset, the sky around the Sun is orangish-red. Why is this? There are two reasons:

1.      The blue light is scattering away from our line of sight so all we see from the Sun are reds and oranges.

2.      The amount of atmosphere the light travels through to reach our eyes is more at sunrise and sunset than when the Sun is directly overhead. More dust (and pollution) for the blue light to interact with and be reflected away from us.