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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.

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Key Facts In This Video

  1. If you shine a light on a perfectly spherical object, you will see a bright dot in the middle of its shadow. 00:33

  2. See an example of a Poisson spot: 05:24

  3. To see a Poisson spot, you need a coherent light source, which is why the spots don't commonly appear in everyday life. 07:04