Biot's law - determines the angle φ through which the plane of polarisation of linearly polarised light is rotated as it passes through a layer of a non-crystalline substance (a liquid, or a solution in an inactive solvent) that possesses natural optical activity: φ= [α]lc, where l is the thickness of the layer of the substance, c is its concentration, [α] — the rotation constant (unlike the rotation constant α for crystals, this coefficient is written in square brackets for solutions). Established by the French physicist J. B. Biot in 1815. Biot's law expresses the proportionality of φ to the number of optically active molecules in the path of the light beam. (see Optical activity).