Definition
A physical law stating that the intensity of a radiated quantity — such as light, sound, or radio signal strength — decreases in proportion to the square of the distance from the source. If the distance from the source doubles, the intensity falls to one-quarter; if the distance triples, the intensity falls to one-ninth.
Plain English
The farther you get from something that radiates outward, the weaker it gets — and it weakens much faster than you'd expect. Doubling the distance doesn't cut the strength in half; it cuts it to a quarter.
Context Anchor
Seen in aviation discussions of radio signal strength, light intensity, and other energy that spreads outward from a source.
Derivation
From Latin 'inversus' (turned upside down) and 'square' (a number multiplied by itself). The name describes the math directly: intensity is inversely related to distance squared. Knowing the name describes the formula makes the rule easier to remember.
Why Pilots Care
Explains why VOR, ILS, or radar signals become unusable beyond certain distances even in line of sight.
Analogy
Think of shining a flashlight on a wall. When you move farther away, the same light spreads over a larger area, so any one spot on the wall receives less light.
Grounding Statement
Picture a single light bulb in a dark hangar: standing 10 feet away it lights your hand clearly, but at 20 feet it looks four times dimmer, not twice.
Intuition Check
Do not assume twice the distance means half the strength. Under the inverse square law, twice the distance means one-fourth the strength.
Example Sentence 1
Because of the inverse square law, a VOR signal received at 40 miles is far weaker than at 20 miles, even though the distance only doubled.
Example Sentence 2
At twice the distance the radar return dropped to one quarter strength following the inverse square law.