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.
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)