page_title: Siméon Denis Poisson
section_title: Optics
hierarchical_section_title: Siméon Denis Poisson / Contributions / Optics
caption_attribution_description: English: The shadow of a circular obstacle (5.8 mm) in diameter lit by a ~0.5 mm sunlight source, exposing the Arago spot in the middle of a shadow. Distance from the light source to the obstacle 5 ft (~153 cm), distance from the obstacle to the paper screen 6 ft (~183 cm).
context_page_description: Baron Siméon Denis Poisson FRS FRSE was a French mathematician, engineer, and physicist who made many scientific advances.
context_section_description: Poisson was a member of the academic "old guard" at the Académie royale des sciences de l'Institut de France, who were staunch believers in the particle theory of light and were skeptical of its alternative, the wave theory. In 1818, the Académie set the topic of their prize as diffraction. One of the participants, civil engineer and opticist Augustin-Jean Fresnel submitted a thesis explaining diffraction derived from analysis of both the Huygens–Fresnel principle and Young's double slit experiment.
  Poisson studied Fresnel's theory in detail and looked for a way to prove it wrong. Poisson thought that he had found a flaw when he demonstrated that Fresnel's theory predicts an on-axis bright spot in the shadow of a circular obstacle blocking a point source of light, where the particle-theory of light predicts complete darkness. Poisson argued this was absurd and Fresnel's model was wrong. (Such a spot is not easily observed in everyday situations, because most everyday sources of light are not good point sources.)
  The head of the committee, Dominique-François-Jean Arago, performed the experiment. He molded a 2 mm metallic disk to a glass plate with wax. To everyone's surprise he observed the predicted bright spot, which vindicated the wave model. Fresnel won the competition.
  After that, the corpuscular theory of light was dead, but was revived in the twentieth century in a different form, wave-particle duality. Arago later noted that the diffraction bright spot (which later became known as both the Arago spot and the Poisson spot) had already been observed by Joseph-Nicolas Delisle and Giacomo F. Maraldi a century earlier.
caption: Photo of the Arago spot in a shadow of a 5.8 mm circular obstacle.
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