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 spatial filter. A small ball
bearing 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, Augustine 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, .010 mm pinhole, small ball bearing mounted on a microscope slide, screen, video camera.
To see more photos of Poisson's Spot witha red laser click here
To see photos of Poisson's Spot with a green laser click here
http://demonstrations.wolfram.com/PoissonSpot/
SETUP TIME: 1 hour. See below:
A Melles-Griot green laser or a Metrologic Red Laser is used.
The technique does not require a beam expander or a lens. It requires a 0.010 mm pinhole. The use of a Tap and Drill Size Gauge from McMaster Carr will suffice as a pinhole.
Note that a .010 sized spatial filter is a very large spatial filter. Most common optical spatial filters are very much smaller.
Use the pinhole to obtain a good pinhole diffraction image. Use a distance of 1.5 to 2 meters between the viewing screen and the pinhole.
The circular object is placed at a approximately in the middle between the screen and the pinhole. The circular object in our case is a small sphere about ~ 2 or 3mm in diameter mounted on a glass slide.
The Poisson's spot is actually an image of the pinhole diffraction!
The interference effect of the edges of the circular disc acts like a long focal length lens. The Poisson spot is an image the light from the pinhole!
A good reference is Arnold Sommerfeld, Lectures on Theoretical Physics, Volume IV, pp. 213 - 221.
Look at Arnold Sommerfeld's Optics, volume IV in his lectures on Theoretical Physics. His discussion on Poisson's spot is amazing.
Sommerfeld discusses how a circular metal disk can be used as a lens to do photography!
A physics laboratory in Moscow used a dinner plate held up by someones hand to get a Poisson's spot on a wall 40 meters away!
Updated by JZ on 12/9/2010