Dominique Francois Jean Arago (1786-1853), French physicist, surveyor, mathematician, astronomer and politician, usually known as Francois Arago. Arago's early survey work led to him being elected a member of the French Academy of Sciences at the notably young age of 23, becoming secretary of the Academy in 1830. From 1809 he was an astronomer at the French Royal Observatory. In physics he studied magnetism and optics. Work on the polarisation of light led to his 1838 proposal of an experiment (carried out in 1850) to show the wave nature of light by comparing its speed in air and water. Arago was also involved in politics, and he helped to abolish slavery in the French colonies in 1848. 19th-century engraving.