Poisson's Spt

Also known as the Arago Spot

 3 Aug 2009 


What is the Poisson Spot? It is a bright point that appears in the geometrical shadow of a circular object illuminated by light from a point source. This spot is of considerable interest to physicists due to the historical part it played back in the wave theory of light. Light, or electromagnetic radiation as we know now, exhibits properties of both waves and particles (known as the wave-particle duality). But back the in the past, this idea was locked in heated debate over the nature of light.

The earliest comprehensive theory of light was put forth by Christiaan Huygens, who proposed the wave theory of light. He also demonstrated how light waves could interfere with each other. However, Huygens' wave theory was overshadowed by Issac Newton's corpuscular theory of light just a few years later in the 1670s. In Newton's theory, light was composed of a great many tiny particles, and he was able to successfully explain the phenomenon of reflection. With some difficulty, he also managed to explain refraction through lenses and splitting of white light using a prism. Given Newton's status, his particle theory was essentially unchallenged for over a century.

... to be continued ...

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