Students investigate in the introductory laboratory the relationship between distance from a light source and illuminance. A graphical display and analysis of data leads to the ’’discovery’’ of the inverse square law. Its validity is confirmed by excellent agreement between predicted and observed values. A 12‐V high‐intensity bulb is used as a point source; the detector is an inexpensive silicon photocell connected to a milliammeter. The assumption of a point source requirement is tested by the use of a diffusing screen.