At sunset it is common to see rays or beams of sun light radiating from the sun. These are called crepuscular rays. The first thing to stress about crepuscular rays is that they are actually parallel beams of light, and not diverging as they appear. This is an illusion caused by perspective. The sun is so far away that the light reaching the earth is fairly well collimated and obstructions such as clouds or mountains cause bright beams of light, or darks shadows, in the atmosphere which are very close to parallel and just appear to converge or diverge. The further away something is, the smaller it looks. This is what causes the edges of a straight road or railway tracks to look like they converge in the distance. This is called perspective.
Crepuscular rays at sunset in SE Queensland Australia
As mentioned above crepuscular rays can be either beams of light or dark shadows in the atmosphere. In the case of beams of light, they can be seen due to the air they pass through scattering light. Thus the more air they pass through the brighter they look. This is why they are more prominent at the horizons and when the sun is low. When looking for crepuscular rays overhead you are looking directly across the beam, not along it, and will see less scattered light. In the case of shadows they can be seen because of all the air around them scattering light.
Crepuscular Rays over Ardery Island in the Windmill Islands
Anti Crepuscular Rays
Crepuscular rays can be up going, down going, even radiating. But less common is to see the rays after they have passed the observer. These are known as anticrepuscular rays because they diverge to the anti solar point. They are much weaker, and thus less commonly observed, because of the large amount of scattering which has already occurred by the time they reach the other horizon, resulting in a lower intensity light.
Anti crepuscular rays converging at the anti solar point