Why is the sky blue?

The Earth's atmosphere scatters incoming light from the sun. This incoming light is made up of lots of different colours. Each colour of light has a different wavelength and because of this, different colours of light scatter at different angles when they hit the atmosphere. The shorter the wavelength of light, the more the light scatters. This means that the sky is filled with the light of shortest wavelengths, violet and blue. However, the cones in our eyes responsible for perceiving colours are more sensitive to the blue part of the spectrum, and so the sky appears purely blue.