Inverse Square Law

The law that states when thermal or nuclear radiation is uniformly emitted from a point source, the amount received per unit area at any given distance from the source, assuming no absorption, is inversely proportional to the square of that distance. 144

The physical law stating that irradiance, i.e., the power per unit area in the direction of propagation, of a spherical wavefront varies inversely as the square of the distance from the source, assuming there are no losses caused by absorption or scattering. Note: For example, the power radiated from a point source, e.g., an omnidirectional isotropic antenna, or from any source at very large distances from the source compared to the size of the source, must spread itself over larger and larger spherical surfaces as the distance from the source increases. Diffuse and incoherent radiation are similarly affected.