Editing Siméon Denis Poisson (section)

=== Optics ===
[[File:A_photograph_of_the_Arago_spot.png|thumb|right|200px|Photo of the Arago spot in a shadow of a 5.8 mm circular obstacle.]]
Poisson was a member of the academic "old guard" at the [[French Academy of Sciences|Académie royale des sciences de l'Institut de France]], who were staunch believers in the [[Wave–particle duality|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]].<ref name="fresnel1868">{{Citation|last=Fresnel|first=A.J.|title=OEuvres Completes 1|url=https://books.google.com/books?id=3QgAAAAAMAAJ|year=1868|publication-place=Paris|publisher=Imprimerie impériale}}</ref>

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, [[François Arago|Dominique-François-Jean Arago]], performed the experiment. He molded a 2&nbsp;mm metallic disk to a glass plate with wax.<ref name="fresnel1868_arago">{{Citation|last=Fresnel|first=A.J.|title=OEuvres Completes 1|url=https://books.google.com/books?id=3QgAAAAAMAAJ|page=369|year=1868|publication-place=Paris|publisher=Imprimerie impériale}}</ref> 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]]<ref name="fresnel1868_arago" /> and [[Giacomo F. Maraldi]]<ref name="maraldi1723">{{Citation|last=Maraldi|first=G.F.|title='Diverses expèriences d'optique' in Mémoires de l'Académie Royale des Sciences|url=http://gallica.bnf.fr/ark:/12148/bpt6k3592w/f300.image.langFR|page=111|year=1723|publisher=Imprimerie impériale}}</ref> a century earlier.