I hear photographers talk about the inverse-square law, particularly with reference to lighting.
What is this law about, and most importantly, how is it applied to lighting for photography?
The law states:
If you double your distance from a light source, the amount of light reaching you drops to a quarter of what it was.
If you multiply your distance from a light source by X, the amount of light reaching you will drop by a factor of X^2 (X squared)
As is often the case, Wikipedia explains this very nicely (with a nice graphic, too).
This means that you don't have to move very much relative to your light source in order to see a big change in the amount of light.
So, if you are lighting a subject with a strobe, you only have to move the strobe a little closer or further away to achieve a big difference in the amount of light reaching the subject.
The inverse square is the proportion between the light intensity and the distance to the light source. At double the distance, the light is one fourth (1/2*2).
This applies to anything that spreads in all directions from a source, as the area of a sphere is proportional to the square of the radius.
So, if you move a lamp/flash away from the model, the light is reduced by the square of the distance. If you move the lamp from 1 meters to 2 meters, the light gets two stops weaker (1/2*2 = 1/4). If you move the lamp from 1 to 3 meters, the light gets a little more than three stops weaker (1/3*3 = 1/9).