Additional Info

• ID Code: M2-03
• Purpose: 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. In the photograph at the left above the alignment of the ball in the expanded laser beam can be seen on the screen. In the photograph at the right the Poisson bright spot is seen on a ground glass screen about twenty feet from the laser, looking back toward the laser beam. This picture can be displayed on a video monitor or using a video projector.

  There is a fascinating story about the origin of this experiment, referenced from Eugene Hecht, Optics (Second Edition) and a the web site Fresnel Diffraction, written by Dean Dauger:

  In 1818, Augustin Fresnel submitted a paper on the theory of diffraction for a competition sponsored by the French Academy. His theory represented light as a wave, as opposed to a bombardment of hard little particles, which was the subject of a debate that lasted since Newton's day. Siméon Poisson, a member of the judging committee for the competition, was very critical of the wave theory of light. Using Fresnel's theory, Poisson deduced the seemingly absurd prediction that a bright spot should appear behind a circular obstruction, a prediction he felt was the last nail in the coffin for Fresnel's theory. However, Dominique Arago, another member of the judging committee, almost immediately verified the spot experimentally. Fresnel won the competition, and, although it may be more appropriate to call it "the Spot of Arago," the spot goes down in history with the name "Poisson's bright spot" like a curse.

  Note that the alignment of this system can be delicate and time-consuming; it is not recommended to combine with with other demonstrations using the same laser in a single 50-minute lecture.

• Availability: Available
• References: REFERENCES: (PIRA 6C20.10)