Wiktionary
n. (context physics English) any physical law, such as that of gravitation, in which a quantity varies inversely with distance from a source as the square of that distance
Wikipedia
In physics, an inverse-square law is any physical law stating that a specified physical quantity or intensity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental cause for this can be understood as geometric dilution corresponding to point-source radiation into three-dimensional space (see diagram). Mathematically formulated:
$$\mbox{Intensity} \ \propto \ \frac{1}{\mbox{distance}^2} \,$$
It can also be mathematically expressed as:
$$\frac{\mbox{Intensity}_1}{\mbox{Intensity}_2} = \frac{{\mbox{distance}_2}^2}{{\mbox{distance}_1}^2}$$
or as the formulation of a constant quantity:
Intensity * distance = Intensity * distance
The divergence of a vector field which 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, and hence zero outside sources. Newton's law of universal gravitation follows an inverse-square law, as do the effects of electric, magnetic, light, sound, and radiation phenomena.
Radar energy expands during both the signal transmission and also on the reflected return, so the inverse square for both paths means that the radar will receive energy according to 1/r power.
In order to prevent dilution of energy while propagating a signal, certain methods can be used such as a waveguide, which acts like a canal does for water, or how a gun barrel restricts hot gas expansion to one dimension in order to prevent loss of energy transfer to a bullet.
Usage examples of "inverse-square law".
The force diminished with distance as if it was being spread out over, respectively, an ever larger two-dimensional surface, producing an inverse-square law, or a four-dimensional hypersurface, yielding a visibly steeper inverse-fourth-power effect.
The old inverse-square law that applies to all force fields, you know.
And this winter had to be dealt with soon--not only did it cool off quickly as the summer faded, but the output of the solar power plant plunged at the same time--we weren't just dealing with the inverse-square law (when the sun became twice as far away, we'd have one-fourth the power), but also more and more cloudy days, lacking weather-control satellites.