Thursday, January 29, 2009
What is the Poisson's Spot?
When a small opaque circular disc is placed in front of a point source of light, it casts a shadow on the screen placed behind it. But, the most amazing observation will be that there will be a bright spot at the center of the geometrical shadow. This spot is called the Poisson's Spot, after the famous physicist Simeon Denis Poisson. Poisson in the early nineteenth century, mathematically derived from the diffraction theory that there should be a bright spot at the center of the geometrical shadow of the disc, which he disliked as it was against an intuition. But when his mathematical results were experimentally verified, he became a strong follower of the wave theory of light.
The formation of the Poisson's spot can be explained using the Huygens' Theory. When light from a point source hits a circular disc, each point on the circumference of the disc acts as a secondary source of light. Since the waves from the points at circumference reach the center of the geometrical shadow in phase, they interfere constructively. As a result of this, the intensity there is a maximum which gives rise to the Poisson's Spot.