Why the Sky Is Blue

Sunlight looks white but is a mix of all colours. As it passes through the air, it bumps into molecules that scatter it in every direction. Here's the key: shorter wavelengths (blue and violet) scatter much more strongly than longer ones (red). So blue light gets bounced all over the sky, and when you look up in any direction, that scattered blue is what reaches your eyes. The sky is basically glowing with re-directed blue sunlight. At sunset the light travels through much more air to reach you. By then most of the blue has been scattered away sideways, leaving the reds and oranges to come straight through. Same physics, different path length. (Violet scatters even more than blue, but our eyes are less sensitive to it and the sun emits less of it, so we see blue.)