Jean Baptiste Biot (1774-1862), French physicist. Biot's most famous work concerned optical activity. He showed for the first time that different types of quartz rotate the plane of polarised light in different directions, later extending this to many other substances. He demonstrated that the amount of rotation is proportional to the path length of light through the medium and to the medium's concentration. In 1804 with Gay-Lussac he made an early balloon ascent and made meteorological and magnetic observations up to 5000 metres.