Étienne-Louis Malus

Étienne-Louis Malus (; ; 23 July 1775 – 23 February 1812) was a French officer, engineer, physicist, and mathematician.

Malus was born in Paris, France and studied at the military engineering school at Mezires where he was taught by Gaspard Monge. He participated in Napoleon's expedition into Egypt (1798 to 1801). He was also a member of the mathematics section of the Institut d'Égypte. Malus became a member of the Académie des Sciences in 1810. In 1810 the Royal Society of London awarded him the Rumford Medal.

His name is one of the 72 names inscribed on the Eiffel tower.

He had been in poor health for several years, partly due to illnesses contracted during Napoleon’s Egyptian campaign. By 1812, his tuberculosis had advanced, and he died in Paris at age 36.

"Discovery" of polarization In 1810, Malus, while engaged on the theory of double refraction, casually examined through a doubly refracting prism of quartz the sunlight reflected from the windows of the Luxembourg palace. He was surprised to find that the two rays alternately disappeared as the prism was rotated through successive right angles, in other words, that the reflected light had acquired properties exactly corresponding to those of the rays transmitted through Iceland spar.

He named this phenomenon polarization, and thought it could not be explained by wave theory of light. Instead, he explained it by stating that light-corpuscules have polarity (like magnetic poles).

Selected works Mémoire sur la mesure du pouvoir réfringent des corps opaques. in Nouveau bulletin des sciences de la Société philomathique de Paris, 1 (1807), 77–81 Mémoire sur de nouveaux phénomènes d’optique. ibid., 2 (1811), 291–295 Traité d’optique. in Mémoires présentés à l’Institut des sciences par divers savants, 2 (1811), 214–302 Théorie de la double réfraction de la lumière dans les substances cristallines. ibid., 303–508

Work

Malus mathematically analyzed the properties of a system of continuous light rays in three dimensions. He found the equation of caustic surfaces and the Malus theorem: Rays of light that are emitted from a point source, after which they have been reflected on a surface, are all normal to a common surface, but after the second refraction they no longer have this property. If the perpendicular surface is identified with a wave front, it is obvious that this result is false, which Malus did not realize because he adhered to Newton's theory of light emission. Malus's theorem was not proved until 1824 by W. R. Hamilton, with Adolphe Quetelet and Joseph Diez Gergonne giving a separate proof in 1825.