PURPOSE: To demonstrate the Poisson (or Arago) bright
spot.
DESCRIPTION: This is one of the keystone experiments that established
that light has a wave characteristic. The middle of the shadow from a dark sphere
shows a bright spot. In this modern version of the experiment a laser beam is first
diffracted through a pinhole that acts as a spacial filter. A small ball
bearing which is glued to a microscope slide. The diffraction pattern with its
central bright spot is viewed on a distant screen at the end of the optical rail.
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.
Reference: Optics (2nd edition) by Eugene Hecht
EQUIPMENT: Laser on optical rail, pinhole, ball bearing mounted on slide, screen, video camera.
SETUP TIME: 1 hour. See below:
