Nothing but blue sky …

Why is the sky blue? The short answer is because the light from the sun is diffused in the atmosphere. But why blue? First we need to define the term scattering. Light Scattering is when the light is forced to deviate from a straight trajectory by one or more localized non-uniformities in the medium through which it passes.

The sky appears blue because, air scatters short-wavelength light in preference to longer wavelengths. Blue have the shortest wavelength of the color spectrum, while red have the longest one.

This explains the blue sky, but what about the red one. During daytime, about 25% of the lights is scattered in the atmosphere. Near sunrise and sunset, most of the light we see comes in nearly tangent to the Earth's surface, so that the light's path through the atmosphere is so long that much of the blue and even yellow light is scattered out, leaving only the red wavelength.