(physics, optics) one of several laws relating two quantities such that one quantity varies inversely as the square of the other. There are laws using this basic principle which apply to magnetism, sound and light. An example of this would be that of the illumination produced on a screen by a point source of light: if the distance between the light source and screen were doubled, the illumination on the screen would be reduced to a quarter of its original intensity; if the distance were trebled, the illumination would be reduced to one-ninth; if the distance were quadrupled, the illumination would be reduced to one-sixteenth, etc. Likewise, the intensity of sound decreases as the distance from its source increases: a bell 10 feet away sounds one-fourth as loud as the same bell 5 feet away; and if 15 feet away, it sounds one-ninth a s loud as when 5 feet away. Applied to radiation, the inversesquare law states that the intensity of radiation decreases in proportion to the square of the distance from its source. —All About Radiation Glossary. Final approval 27.9.89