inverse-square law
The principle in physics that the effect of certain forces on an object varies by the inverse square of the distance between the object and the source of the force. The magnitude of light, sound, and gravity obey this law, as do other quantities. For example, an object placed three feet away from a light source will receive only one ninth (132, the inverse of 3 squared) as much illumination as an object placed one foot from the light.
The American Heritage® Science Dictionary
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