Inverse Square Law - Overhead Projector And Two-Meter Stick
To show that light obeys the inverse square law.
Overhead projector, two-meter stick.
With the lens one meter from the screen the area lit by the projector beam is measured. Moving the lens back to two meters from the screen, the width and height of the lit area both double, so the area is four times as great. The intensity is one-fourth for a distance of twice. This demonstrates that the inverse square law has as much to do with geometry as with wave propagation.
Simon George and Robert Doebler, Rainbow Glasses and the Inverse Square Law, TPT 32, 110-111 (1994).