The inverse-square law generally applies when some force, energy, or other conserved quantity is evenly radiated (see flux) outward from a point source in three-dimensional space. Since the surface area of a sphere (which is 4πr2 ) is proportional to the square of the radius, as the emitted radiation gets farther from the source, it is spread out over an area that is increasing in proportion to the square of the distance from the source. Hence, the intensity of radiation passing through any unit area (directly facing the point source) is inversely proportional to the square of the distance from the point source. Gauss' law is similarly applicable, and can be used with any physical quantity that acts in accord to the inverse-square relationship.