A sunset showing Rayleigh scattering
Image courtesy of Fir0002 at en.wikipedia
The explanation for the reddish tints seen in the sky at either end of the day lies in Rayleigh scattering. As it passes through layers in the atmosphere, shorter wavelength light is scattered more than longer wavelength light.
Sunlight is made up of all the colors of the visible light spectrum – red, orange, yellow, green blue, purple, which may seem familiar as all the colors of the rainbow.
Rainbows appear when the different wavelengths get broken up through refraction and scattering in the atmosphere. Basically, as the light passes through different densities in the air, the light bends, kind of like it does in a prism and bounces off tiny particles that are in the air. This also explains why, during the day, the sky is blue. The particles that are in the atmosphere scatter sunlight so that the blue wavelengths are not absorbed, but reflected.
At sunset the same things happen, but because of the angle of the sun and the volume of gas and therefore the density of gas that the light passes through changes, the sky takes on a reddish glow from the longer wavelengths in the visible light spectrum.
The British physicist Lord Rayleigh (1842–1919) was the first to come up with the reasoning behind why the sky is blue, which is why the phenomenon is known as Rayleigh scattering.