The sky appears blue for a combination of two reasons. Before white light reaches the Earth's surface, the light waves collide with and bounce off of the nitrogen and oxygen atoms in the atmosphere. Different frequencies of light (in other words different colours) are scattered differently. Higher frequencies (blue and purple) are more easily scattered and thus bounce around in all different directions more than lower frequencies (red or orange) do. The scattering of high frequencies alone would cause the sky to appear blue and purple, but our eyes work better at frequencies near the middle of the spectrum (yellow and green). Since the colour blue is closer to yellow or green than purple is, the sky we see appears blue.