A viewing frustum is the region in the modeled world that you can see at different distances away from your eye. In other words, it tells what direction a light ray comes in will contribute the image.

Question:

Does it mean that if a light travels at a direction that doesn't fit the viewing frustum, the light won't contribute to the image? Will the light refract at the viewport of the viewing frustum just like light refracts at the surface of our eyes?

Thank you very much!

A viewing frustum does not tell what direction a ray comes in, it merely identifies the volume that is visible.

In general, light volumes(the space that the light can reach) that don't intersect with the view frustum do not contribute to the scene. As well, there is no refraction of light due to the frustum(refraction is a result of 2 different media interfacing).

so:

a point light, with a maximum range of d, will not contribute to the scene unless it is within d units of an object in the frustum.

a spot light, with a range of d, and a direction ld, and a radius of r, at d, along ld, will not contribute light unless the cone described by d,ld,and r intersects with an object in the frustum.

a directional light, used usually to represent the sun in exterior environments, generally affects every object in the scene as it has no position or falloff. However, objects not in the frustum are generally culled, so they are only applied to the objects in the frustum.