Biot's law

Biot's law is a law expressing the proportionality of the rotational power of a medium to the concentration of optically active products (dextrorotatory and levorotatory):

a = a.C.l

where a is the rotation of the plane of vibration of the light waves, a the specific rotation, C the concentration, l the length of the cell (the length of the optical path in the medium).

This law is the foundation of polarimetry, an analytical chemistry technique for measuring the concentration of a solution in optically active substances.