Intensity ∝ 1/distance2
The divergence of a vector field is the resultant of radial inverse square law fields with respect to one or more sources is everywhere proportional to the strength of the local sources. Therefore this zeros outside sources. Properties that follow an inverse square law include, Newton’s law of universal gravitation, electric, magnetic, light, sound and radiation phenomena.
Generally, the inverse square law applies when some force, energy or other conserved quantity is radiated outward radially in three-dimensional space from a point source. The surface area of a sphere is proportional to the square of the radius, as the emitted radiation gets further from the source, it is spread out over an area what is increasing in proportional to the square of the distance from the source.
The intensity of light or other linear waves 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 one-quarter the energy. Generally, the intensity of a spherical wave front varies inversely with the square of the distance from the source.
The inverse square law is used in photography and theatrical lighting. It is used to determine the fall off or the difference in illumination on a subject as it moves closer or further from the light source.