The further the subject is away from the camera, the smaller it's image.
A smaller image contains less light than a larger image.
(Throw on a longer lens, and the image gets bigger. But also darker.
Put a faster lens on, and the image will get brighter, but only because the lens has a larger image collecting surface, so captures a larger solid angle cone eminating from the subject.)
Reflected light does behave differently only in as far as the reflection could be (very) directional. A concave mirror, for instance, does not spread light, but does the opposite. So stand in its focus, and you'll find out why it's called focus.
But generally things don't reflect like that, and the inverse square law isn't a bad description of how also reflected light behaves.
Last edited by Q.G.; 02-08-2011 at 08:23 PM. Click to view previous post history.