The previous comments about scattering are quite true: shorter wavelengths are scattered more strongly. But shouldn't then the sky be violet? After all, the light with the shortest wavelength in the visible spectrum is violet light!

There are two reasons why the sky is blue rather than violet. The first is the solar spectrum. It tails off in the violet, so the incoming intensity of the blue range of the spectrum is higher than of the violet range. Therefore, although violet light is scattered more than blue light, it is also weaker to start with.

The second reason is the human eye. It is less sensitive in the violet range of the spectrum than in the blue range. This effect again "dampens" the violet in our perception and helps to make the sky blue and not violet.

Interestingly, while many people ask why the sky is blue, very few then ask the second question why it isn't violet - although this question seems quite obvious when you read the explanation about scattering of short wavelengths... This question also illustrates the interesting feature of science that every answer to a question quickly leads to more questions - science is a continuous process of new discoveries.

Stephan Matthiesen
http://www.stephan-matthiesen.de