The Inverse Square Law states that any proton, electron, etc. from space diminishes as it gets further from its source. This law is also used when determining how light, for instance, is spread over four times the area if the light source is twice as far away, thus lighting a subject at 1/4th the intensity. You can also think of it as the lamp-to-subject distance when working with lights on a set. Remember: when doubling the distance, 1/4 amount of light will be affected.
The Inverse Square Law is used to measure point light, meaning light that is emitted from a single source. Fluorescent tube lights do not apply.
Since knowing various ways to manipulate light will help you in the long run, especially if you’re limited to certain resources, understanding the Inverse Square Law may just save your day. For instance, if you don’t have dimmers or a scrim on hand to diminish the light, simply use the Inverse Square Law to help you determine the best method to light your subject. (Given you have the space to work with!)
For example. Say for instance, you have a light source that specifies an accurately exposed reading with an f-stop reading of f8. You need 2 f-stops less light to fall on your subject. (to about a f4) What do you do by using the Inverse Square Law? Now, assuming you don’t have access to modifiers such as scrims or nets, and you can’t change the aperture or the shutter speed of the camera lens because it will ruin the effect you want, you can put the Inverse Square Law to use. Therefore, moving the light source (or subject) is your answer.
Moving the light back by twice the distance (from subject) = difference of 2 f-stops = 4 X’s less light or 1/4 less light. (Keep in mind it is a difference of 1 f-stop when working with a spotlight or more directional light sources!)
So, if you’re needing 2 f-stops of less light, simply move your light source back at doubled the distance. Or, you may choose to move your subject back at the same distance (if you do this the other light source--if you have one--will need to be moved as well).
Please be aware that many other factors will affect or determine the look you’re wanting when applying the Inverse Square Law, such as spot lights, flood lights, brightness, contrast you're achieving, etc.) Also note that the Inverse Square Law will have a different fall-off rate for light sources that have attached grids, and softboxes, and fresnels.