Malus, Etienne Louis (1775-1812)

French physicist who discovered the polarization of light by reflection from a surface (see polarized light). He also found the law of polarization that relates the intensity of the polarized beam to the angle of reflection.

Malus was born in Paris and studied there at the Ecole Polytechnique. From 1796 he was in the army, taking part in Napoleon's campaign in Egypt and Syria in 1798 and eventually rising to major, but he was also an examiner for the Ecole Polytechnique.

Malus began doing experiments on double refraction in 1807. This phenomenon causes a light beam to split in two on passing through Iceland spar and certain other crystals. An empirical description had been given by Dutch physicist Christiaan Huygens, based on the assumption that light is wavelike in character, and Malus's results confirmed Huygens's laws.

In 1808, Malus held a piece of Iceland spar up to some light reflecting off a window. To his surprise, the light beam emanating from the crystal was single, not double. He then noted that of the two beams that normally emerge from the crystal, only one was reflected from a water surface if the crystal was held at a certain angle. The other passed into the water and was refracted. If the crystal was turned perpendicular, the second beam was reflected and the first refracted. He described the light as being ‘polarized’.