Inverse-Square Law of Propagation

As electromagnetic radiation leaves its source, it spreads out, traveling in straight lines, as if it were covering the surface of an ever expanding sphere. This area increases proportionally to the square of the distance the radiation has traveled.

The inverse-square law generally applies when some force, energy, or other conserved quantity is evenly radiated 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.

Electromagnetic waves are oscillating electric and magnetic fields that travel through space and carry energy. The inverse square law is directly applicable to electromagnetic waves, explaining how the intensity of these waves decreases as they propagate away from their source. In radio the Inverse-Square Law of Propagation applies to a lossless spherical propagating wave from an omnidirectional radiator and is a factor in describing how power decreases as distance from the source increases although other factors do apply to surmise the true relationship of power dissipation loss and resulting actual levels as they relate to distance.

The inverse square law is a fundamental principle that helps in the understanding of the behavior of electromagnetic waves in various contexts. It explains the way the intensity of these waves decreases as they propagate away from their sources. As a practical simplified example, if you double the distance from the source, the intensity of the phenomenon decreases to one-fourth of its original value. If you triple the distance from the source, the intensity decreases to one-ninth of its original value.