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.


Bamadev said...

Interesting phenomenon! I enjoyed reading it even though I can not make any comments as the theory behind it is beyond of my comprehension.

RKafle said...

Thanks for your comment ! Diffraction phenomenon is simply the bending of waves round corners, edges, etc. It depends upon the wavelength of the waves and the size of the diffracting elements. For example, we can hear the sound from a friend behind a wall although we can't see him/her. The reason is that the sound waves have longer wavelength and so they are diffracted by the top/edge of the wall; on the other hand, the light waves have very short wavelength in comparison to the edge of the wall and so no diffraction can be observed there. Similar kind of thing happens when there is a point source of light in front of a small circular disc.

I think this simplifies the physics that was written in my posting!