Poisson tried to sink the wave theory of light and proved it instead
In 1817 Siméon Denis Poisson sat on a prize committee judging Augustin-Jean Fresnel's paper on diffraction. Sure that light was made of particles, he calculated that Fresnel's waves implied a bright spot at the centre of a round shadow, an absurdity that should sink the theory. Then someone actually tested it.
The committee's head, François Arago, stuck a metal disk two millimetres across onto a glass plate with wax and shone light at it. The bright spot appeared right where the calculation said it would. Fresnel won the prize, the wave model gained a powerful ally, and the feature became known as the Poisson spot, although Joseph-Nicolas Delisle and Giacomo Maraldi had already seen it roughly a century before.
That episode fits a pattern: Poisson's readings of physics were often overturned, while his mathematics lasted. His equation for the potential, first published in 1813, extended Laplace's work and applied to gravity, electricity and magnetism alike, and it inspired George Green's 1828 essay on electrical theory. He kept treating electricity and magnetism as separate even after Ørsted linked them in 1820, yet his magnetic equation still held up.
Talent showed early. Born on 21 June 1781 in Pithiviers, about 80 km south of Paris, the son of a retired soldier, he came first in the entrance exam for the École Polytechnique at 17. Classmates visited his room after hard lectures to have him explain them. Two memoirs written before he was 19 so impressed his professors that he graduated in 1800 without sitting the final exam, and Laplace came to regard him almost as a son.
He succeeded Joseph Fourier as professor in 1806, published over 300 works, and reportedly claimed that life offered just two worthwhile pursuits: working on mathematics and teaching it. Made a baron in 1825, he never used the title. After the 1830 revolution Arago arranged a dinner where King Louis-Philippe greeted him warmly, ending talk of stripping his honours. He died at Sceaux on 25 April 1840 and is among the 72 names engraved on the Eiffel Tower.
Source: Siméon Denis Poisson
