SIMPLE DEMONSTRATION OF THE 1/R^2 LAW FOR INTENSITY VS. DISTANCE:
Projector for illumination of fluids experiments and beam generation:
Use a slide projector with a circular or square aperature to show that radius or side-length increases linearly with distance, so area increases quadratically with distance. Since no photons leave the beam, energy/sec is a constant. Thus intensity = energy per unit area per second ~ 1/area ~ 1/r2 .
Insert square or circular slide into the projector. Project onto white cardboard. Walk a calculated distance and measure side-length (e.g. area). Note how area depends on distance.
Tilt the white cardboard sheet to show how intensity various with latitude (because as sheet is tilted area over which the beam impacts increases).
©2000. John Hart, University of Colorado