For isotropic propagation in three spatial dimensions, a conserved flux from a point source decreases as the reciprocal square of distance. A source of luminosity produces flux .
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The inverse-square law is a fundamental principle in physics that describes how the intensity of a physical quantity decreases as the distance from the source increases. Specifically, it states that the intensity of such a quantity is inversely proportional to the square of the distance from the source. This means that if you double the distance from the source, the intensity becomes one-fourth as strong.