A method of chemical analysis based on the optical activity of the substance being determined. Optically active materials are asymmetric; that is, their molecules or crystals have no plane or centre of symmetry. These asymmetric molecules can occur in either of two forms, d- and l-, called optical isomers. Asymmetric substances possess the power of rotating the plane of polarisation of plane-polarized light. Measurement of the extent of this rotation is called polarimetry. Polarimetry is applied to both organic and inorganic materials.