The Inverse Square Law applies to Radiation. It states that the intensity of any radiation source is inversely proportional to the square of the distance from the source.
I would rather call it “an inverse square law”. Some things vary inversely as the square of something (usually distance) and some things don’t.
For instance, the quantity of light energy per second falling on a tiny area* is inversely related to the square of the distance from the source if a point light source emits light uniformly in all directions. The energy per second will decrease by four times with every twofold increase in distance for a certain area, and nine times with every threefold increase in distance. (Ideally, the area should be a sphere centered on the point source to make the distance clear; nonetheless, an approximate solution is a tiny plane area perpendicular to the line from the point source.) Therefore, light obeys the inverse square law (at least in Euclidean space).
The gravitational and electrical forces are some examples of entities that adhere to the inverse square law. On the other hand, gravitational and electrical potentials adhere to an inverse rule, whereby the potential must be reduced by a factor of two times the distance.
Thank you,
Eddie
