Inverse-square law

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In physics, an inverse-square law is a physical law stating that some quantity is inversely proportional to the square of the distance from a point. In particular,

the gravitational attraction between two massive objects, in addition to being directly proportional to the product of their masses, is inversely proportional to the square of the distance between them; this law was first identified by Isaac Newton;
the force of attraction or repulsion between two electrically charged particles, in addition to being directly proportional to the product of the electric charges, is inversely proportional to the square of the distance between them; this is Coulomb's law;
the intensity of light radiating from a point-source is inversely proportional to the square of the distance from the source. An object twice as far away, receives only 1/4 the energy. More generally, the 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). For example, Sol "provides" 9140W at the distance of Mercury (0.387AU); but only 1370W at the distance of Earth (1AU)—a three-fold increase in distance results in a nine-fold decrease in power.

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. Source: from Federal Standard 1037C