Why is the sun so red at sunset?

This is due to the same phenomenon that causes the sky to be blue: Rayleigh scattering allows blue light to disperse more quickly in the atmosphere. When the sun sets, the light is reflected at such a low angle that it passes through more air on its way to us. The light travels through the atmosphere "at an angle," resulting in a longer distance travelled.

If light passes farther into the atmosphere, more blue light reaches out in all directions. Sunlight is made up of all possible colours, which combine to form white light. The gap between the sun and the ground at sunset (and sunrise) is so great that almost all blue light is dispersed in various directions.

The red light has the toughest time continuing straight ahead in the same direction through the atmosphere of all the colours. Only red light passes through as the sun is nearest to the horizon, making the sun appear red.