Fresnel, Augustin Jean (1788–1827)

French physicist who refined the theory of polarized light. Fresnel realized in 1821 that light waves do not vibrate like sound waves longitudinally, in the direction of their motion, but transversely, at right angles to the direction of the propagated wave.

Fresnel first had to confirm the wave theory of light. He demonstrated mathematically that the dimensions of light and dark bands produced by diffraction could be related to the wavelength of the light producing them if light consisted of waves. To explain double refraction, he then arrived at the theory of transverse waves.

Fresnel was born in Broglie, Normandy, and studied in Paris, becoming a civil engineer for the government. When Napoleon returned to France from Elba in 1815, Fresnel deserted his post in protest. He was placed under house arrest, taking advantage of this enforced leisure to develop his ideas on the wave nature of light into a comprehensive mathematical theory. Napoleon's return proved to be short-lived and Fresnel was reinstated into government service. Fresnel applied his new ideas on light to lenses for lighthouses. He produced a revolutionary design consisting of concentric rings of triangular cross-section, varying the overall curvature to produce lenses that required no reflectors to produce a bright parallel beam.