Inverse Square Law

[XL | t++ | ***]  inverse square law, luminosity

What it shows:  The intensity of light from a point source decreases as 1/r2, where r is the distance from the source.

How it works:  For the point light source, we use a 1500 watt clear light bulb. The detector is a small solar panel.1 The output current is directly proportional to the intensity of the light falling on the panel and the current is displayed on an analog milliameter. (The current can also be measured by a digital meter or computer.) Measured currents are 8.0, 2.0, and 0.9 mA at distances of 2, 4, and 6 meters, respectively. It works remarkably well! A 5 mA analog meter is used (because we don't have a 10 mA meter) with a 3.3 ohm shunt across its input. This divides the current by two and the meter reads 4.0, 1.0, and 0.45 mA at the prescribed distances.

Setting it up:  The light detector remains fixed in position. A video camera is needed to show the meter's display to the audience. The distance is varied by moving the light source on a cart. With chalk, mark off 2, 4, and 6 meters on the floor.

As a quantitative experiment, this demonstration works remarkably well, even with the room lights on, as long as the light source is not near a wall or other reflecting surfaces.


1.  6 Volt 2 watt solar cell panel from  It measures approximately 5"x4"