Poisson's Spot

Diffraction produces a bright spot where Poisson believed there would be darkness.

Poisson's Spot

What It Shows

Edge diffraction around a 1/8" diameter steel ball bearing results in a visible spot in the center of its shadow. In 1818 this result—to the chagrin of Siméon D. Poisson—agreed with Augustin-Jean Fresnel's prediction, and finally convinced the French Academy of the wavelike nature of light.

Setting It Up

Laser light passes through a 10cm lens. Just beyond the focal point the ball bearing is magnetically suspended from the point of a sewing needle (the needle is taped to a short steel post with small magnets attached to it). The cone of light emerging from the focus should just barely envelop the ball—this happens at about 12cm from the lens.

The placement of the ball is crucial to this experiment and can take several minutes. When possible the elements should be aligned in advance and their positions on the rail should be marked with masking tape if they need to be moved (often this demo is the last in a succession of diffraction demos for a single class meeting). We only have a few ball bearings, so when handling them it's a good idea to hold a sheet of paper underneath so they don't fall on the floor.


Because of the subtle size and brightness of the spot we usually set up the laser in the back of the lecture hall and project the pattern onto the big screen. For best results turn off all the lights in the hall and give the audience's eyes a moment to dark-adapt.

As an added bonus diffraction around the suspending pinpoint is also visible.