The Poisson Spot Puts Light Inside A Shadow
The Poisson spot is also called an Arago spot or a Fresnel bright spot. All three names are those of men who were part of a competition in 1818 that meant to determine whether light was a particle or a wave. Augustin-Jean Fresnel proposed that it was a wave, but Siméon Denis Poisson, a judge in the competition, rejected that theory. He claimed that if light was a wave, then they would observe a bright spot in the shadow of a circular object-a ridiculous idea. The head of the competition committee, Dominique-François-Jean Arago, then decided to test this "absurd" conclusion, and did indeed find the spot. Fresnel won the competition.
Check Out This Month's Most Popular Topic
Key Facts In This Video
If you shine a light on a perfectly spherical object, you will see a bright dot in the middle of its shadow. (0:33)
See an example of a Poisson spot: (5:24)
To see a Poisson spot, you need a coherent light source, which is why the spots don't commonly appear in everyday life. (7:04)