Malus’ malady

French physicist and mathematician Étienne-Louis Malus was originally to have studied at an engineering school but, following the French Revolution, the school was suppressed and he temporarily became a manual labourer. The engineer overseeing his work noted that Malus had developed an approach to the project that minimised the fatigue of the other labourers. Realising his potential, the manager arranged for him to attend the École Polytechnique.

After his studies, Malus joined the French Army and was stationed in Giessen in Germany where he fell in love with the daughter of the chancellor of the university. But, before they were able to marry, Malus received orders to move to Toulon to join an expeditionary force. During fighting in the Middle East in Jaffa, the French forces were overrun and many officers were killed. However Malus escaped death because he had fallen asleep in a trench.

As the war went on, Malus caught the plague and wrote his first book on optics whilst convalescing in a hut with a palm-leaf roof. The physicist Arago (also himself no stranger to adventure) commented:

“No army in the world ever before counted in its ranks an officer who occupied himself in the spare hours of advanced posts with researches so complete and so profound.”

Malus discovered that light could be polarised by reflection through a chance observation. From his home in the Rue d’Enfer, Paris, he used a crystal of Iceland spar (a transparent calcite) to observe light reflected from the windows of the Luxembourg Palace and noted that the two images produced changed in intensity as he rotated the crystal.

In contrast to his eponymous law (which describes the relationship between the transmitted intensity of a beam of plane-polarised light and the orientation of a polariser it passes through) some of Malus’ other ideas on light were less wellfounded. He suggested that light was composed of a combination of caloric (the hypothetical fluid that was proposed to explain heat flow) and oxygen. He also argued that colours depended on the ratio of these two components and that red light had a greater heating effect because it contained more caloric.