The inverse square law is the intensity of light from a constant source falls off as the square of the distance from the source. Any light source that spreads light in all directions, i.e. a lightbulb obeys this law. If you were to walk away from a campfire in the middle of a dark woods, it would get dark pretty quickly. Basically, the double the distance you are from the light, you quarter the light intensity. The light falls off 1 over the distance multiplied by itself. The light measured at 2 metres from the source will be half squared the intensity, if it was one meter it would be a quarter of the intensity. a light measured 4 metres from the same source would be a quarter squared or a sixteenth the intensity at one metre. In photography, as every stop means halving or doubling the light, a quarter of the light is two stops down and a 16th of the light is four stops down. A light reading of f/16 at 1 metre, for example would be f/8 at 2 metres and f/4 at 4 metres. The only light source that conflicts with this law is the sun, as the distance we move something on earth is quite trivial in comparison to the distance between the earth to the sun.