Fresnel , Augustin Jean

(1788–1827) French physicist

Fresnel was born in Broglie, France, and grew up in the time of the French Revolution; by the time he was 26, Napoleon had been exiled and Louis XVIII was on the throne. At this time Fresnel was a qualified engineer but, when Napoleon returned from Elba, Fresnel supported the royalists and lost his job as a result.

Fresnel started studying optics in 1814 and was one of the major supporters of the wave theory of light. He worked on interference, at first being unaware of the work of Thomas Young, and produced a number of devices for giving interference effects.Fresnel's biprism is a single prism formed of two identical narrow-angled prisms base-to-base. Placed in front of a single source it splits the beam into two parts, which can produce interference fringes. Initially, Fresnel believed that light was a longitudinal wave motion, but he later decided that it must be transverse to account for the phenomenon of polarization.

Another important part of Fresnel's work was his development of optical systems for lighthouses. He invented the Fresnel lens – a lens with a stepped surface – to replace the heavy metal mirrors that were in use at the time.

Fresnel became a member of the French Academy of Sciences in 1823 and four years later, shortly before he died, the Royal Society awarded him the Rumford medal.