The inverse square law of intensity (where intensity (1)/(r^2) )...

For a point source, the power $P$ is distributed uniformly over a spherical surface of radius $r$.

The surface area of the sphere is $A = 4\pi r^2$.

Intensity $I = \dfrac{P}{A} = \dfrac{P}{4\pi r^2}$.

Thus, $I \propto \dfrac{1}{r^2}$, which is the inverse square law.

For a line or cylindrical source, the power is distributed over a cylindrical surface, so $I \propto \dfrac{1}{r}$.

For a plane source, the intensity is independent of distance.