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Biot, Jean (Baptiste)

light plane rotation molecular

[beeoh] (1774–1862) French physicist: pioneer of polarimetry.

A child during the French Revolution, Biot joined the artillery at 18 but soon left to study mathematics, and at 26 was teaching physics at the Collège de France. His research showed variety. With he made an early balloon ascent (1804) and made meteorological and magnetic observations up to 5 km; his nerve failed for a second attempt. He made a number of geodetic and astronomical expeditions, visiting Spain and Orkney.

His famous work is on optical activity. He showed, for the first time, that some crystals of quartz rotated the plane of polarized light while other crystals rotated it to the same extent but in the opposite direction. In 1815 he showed that some liquids (eg turpentine) will also rotate plane polarized light, and later he observed the same effect with some solids when dissolved in water (eg sugar, and tartaric acid). He realized that this ability of some substances in solution must mean that the effect is a molecular property (‘optical activity’). He showed (Biot’s Law) that the amount of rotation of the plane of polarization of light passing through an optically active medium is proportional to the length of its path, and to the concentration, if the medium is a solution of an active solute in an inactive solvent, and that the rotation is roughly inversely proportional to the square of the wavelength of the light.

Polarimetry (measurement of optical activity) was pioneered by Biot and after 1870 proved of great value in gaining information on molecular configuration (shape). Later still, the variation in optical rotation with the wavelength of the polarized light was also found to be useful in locating molecular shape.

Biot was a man of great talent, as was seen by his older friend ; and in his old age Biot saw and appreciated the talent of his young friend .

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