The inverse Square Law is something that i have been reminded of quite frequently as a photographer, and should definitely be remembered whenever you are shooting with anything other than ambient light. Studio work especially! A lot of information and detailed explanations can be found online.

S = Source

r = measured points

This diagram shows the spread of light over distance. The further away you are from the light source the more spread out the light will be, meaning that you will get a more even light that is darker than that of light that is closer to the source. depending on how the light is being diffused will change how the inverse square law operates. honey comb grids will still abide by the inverse square law for example for the light fall off over distance will be much lower because the light is being controlled and directed.