PURPOSE: To demonstrate the Poisson (or Arago) bright spot.
DESCRIPTION: 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 a sphere illuminated by a monochromatic light beam. Poisson claimed that was ridiculous. When it was actually discovered by Arago, Poisson then took credit for the discovery!
REFERENCES: (PIRA 6C20.10)
EQUIPMENT: Laser with beam expander/collimator and mounted steel ball bearing with screen.
SETUP TIME: 10 min.
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