Sunday, May 10, 2009

May 10: Augustin-Jean Fresnel

Augustin-Jean Fresnel
May 10, 1788 – July 14, 1827

Augustin-Jean Fresnel was a French physicist who contributed significantly to the establishment of the theory of wave optics. Fresnel studied the behaviour of light both theoretically and experimentally. He is perhaps best known as the inventor of the Fresnel lens, first adopted in lighthouses while he was a French commissioner of lighthouses, and found in many applications today.

His researches in optics, which continued until his death, appear to have begun about the year 1814, when he prepared a paper on the aberration of light, which, however, was not published. In 1818 he wrote a memoir on diffraction for which in the ensuing year he received the prize of the Académie des Sciences at Paris. He was in 1823 unanimously elected a member of the academy, and in 1825 he became a member of the Royal Society of London, which in 1827, at the time of his last illness, awarded him the Rumford Medal. In 1819 he was nominated a commissioner of lighthouses, for which he was the first to construct a special type of lens, now called a Fresnel lens, as substitutes for mirrors.

His labours in the cause of optical science received during his lifetime only scant public recognition, and some of his papers were not printed by the Académie des Sciences till many years after his death. But as he wrote to Young in 1824: in himself 
"that sensibility, or that vanity, which people call love of glory" had been blunted. "All the compliments," he says, "that I have received from Arago, Laplace and Biot never gave me so much pleasure as the discovery of a theoretic truth, or the confirmation of a calculation by experiment."
His discoveries and mathematical deductions, building on experimental work by Thomas Young, extended the wave theory of light to a large class of optical phenomena. In 1817, Young had proposed a small transverse component to light, while yet retaining a far larger longitudinal component. Fresnel, by the year 1821, was able to show via mathematical methods that polarization could be explained only if light was entirely transverse, with no longitudinal vibration whatsoever.

His use of two plane mirrors of metal, forming with each other an angle of nearly 180°, allowed him to avoid the diffraction effects caused (by the apertures) in the experiment of F. M. Grimaldi on interference. This allowed him to conclusively account for the phenomenon of interference in accordance with the wave theory.

With François Arago he studied the laws of the interference of polarized rays. He obtained circularly polarized light by means of a rhombus of glass, known as a Fresnel rhomb, having obtuse angles of 126° and acute angles of 54°.

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