Poisson's spot: Interference pattern within circular shadow.

Poisson's spot: Interference pattern within circular shadow. In the 1820s, Poisson noted that if Fresnel's wave theory of light were correct, then light from a point source falling on a solid disk should constructively interfere at the center of the disk's shadow, leaving a bright spot. In this setup, a monochromatic he-ne laser beam (aimed with the aid of a goniometer) is expanded by a -4" lens and hits a small ball bearing mounted on a glass plate. The image is then expanded by a -10" lens and projected on a screen. The bright spot at the center of the shadow is very evident.