Light also 'works' in accordance with the inverse square law when coming from extended light sources. It can't do anything else.
An extended light source is nothing other than a collection of point light sources. The distance from the object to each of those points in the light source will be different, so it gets complicated because of that: the total amount of light an object receives is the sum of what it receives from every point in that extended light source. But no matter from what point it comes, light will follow the inverse square law.
Unless, of course, the distance to the extended light source is large enough to make the differences in distance be so small that they do not matter.
Then you can ignore the complexity and do simple math again. And still that inverse square thing applies.
A good example of such an extended light source is the sun. Even from here, one astronomical unit away, it has an appreciable size. But that matters so little that it can be completely ignored.
Only when light is directional enough will the inverse square law not apply. Lasers, for instance, do spread, but not in such a way that the inverse square law would be applicable.
Last edited by Q.G.; 02-08-2011 at 08:53 PM. Click to view previous post history.