Augustin-Jean Fresnel

Augustin-Jean Fresnel was born in Broglie, in Normandy, in 1788 — the year before the French Revolution. He was a slow speaker as a child, barely literate until the age of eight, and was orphaned young. He trained at the École Polytechnique and then the Ponts et Chaussées as a civil engineer, working on road and bridge projects across rural France for most of his twenties. The pivot came in 1815: after Napoleon's return during the Hundred Days, Fresnel — who had sided with the monarchy — was dismissed from his engineering post when the Bourbon restoration failed to protect its own supporters. Unemployed in the countryside, with nothing to do, he started doing serious optics. His earliest papers presented a mathematical wave theory of light, directly confronting the Laplacean particle-faction consensus at the French Academy. Arago read the papers, recognised their importance, and the two became lifelong collaborators — Arago protecting Fresnel politically while Fresnel produced the mathematics.

The decisive moment came in 1818. The Academy had posed a prize competition on the theory of diffraction, expecting the particle faction to win. Fresnel submitted a wave-theory derivation — the Fresnel integrals for diffraction of a wave around an obstacle. Poisson, sitting on the committee and hostile to waves, worked out a consequence: if Fresnel's theory was right, a circular obstacle should cast a shadow with a *bright spot* at its exact centre, where all the diffracted wavelets arrived in phase. This was "obviously absurd," Poisson argued, and therefore the wave theory was wrong. Arago performed the experiment: the spot was there. It is now called the Poisson spot — a name applied ironically to someone who never wanted to be associated with it. Wave theory won. Over 1821–1823 Fresnel extended his analysis to derive the amplitude coefficients for reflection and transmission at dielectric interfaces — the four *Fresnel equations* that every optical engineer now uses to design anti-reflective coatings, polarising beam splitters, and laser-cavity Brewster windows.

His other enduring contribution was the Fresnel lens: a design using concentric prism rings that replaced the massive, optically flawed solid lens of earlier lighthouses with a thin, flat, composite optic. The first first-order Fresnel lens was installed at the Cordouan lighthouse in 1822; its beam was visible 32 km out to sea, a range that no prior lighthouse had achieved. Every major lighthouse in the world for the next century used a Fresnel lens; modern solar concentrators and camera flashes still do. He died of tuberculosis in 1827 at thirty-nine. The Royal Society's Copley Medal — which he had been awarded for the wave-theory work — arrived from London a few weeks before his death. Arago delivered the eulogy.