Laser Diffraction - Poisson'S Bright Spot


To demonstrate the Poisson (or Arago) bright spot.


Laser with beam expander/collimator and mounted steel ball bearing with screen.

Setup Time

10 min




This is one of the keystone experiments in establishing light as a wave (rather than particles). The laser beam is expanded to around 4 cm diameter and passed around a 1 inch ball bearing which is suspended between two pointed rods. The diffraction pattern with its central bright spot is viewed on a distant screen or using a small screen on the end of the optical rail. ◙There is a fascinating story about the origin of this experiment, the details of which I don't remember, but which goes something like this: Fresnel claimed that if light were a wave there would be a bright spot on the axis of


Robert P. Barrett, Letter: Poisson's Circles?, TPT 332, 326 ((1994). ◙Wallace A. Hilton, Arago White Spot, AJP 36 #4, ix-x (1968). ◙John B. Johnston, Projecting Poisson's Spot, TPT 16, 179-180 (1978). ◙T. Kallard, Exploring Laser Light, Demonstrating the Paths of Diffracted Rays, pp. 184-185. ◙C. Harvey Palmer, Optics Experiments and Demonstrations, Experiment B13. Demonstration of Fresnel Diffraction by Circular◙Apertures and Obstructions, pp. 196-198.