When light vibrating in all directions is passed through a polarizer filter, the emerging light (polarized light) vibrates in only one direction. Some substances in its path can turn this direction of light towards the right or left. Molecules which rotate the plane of polarized light are said to be optically active. Molecules in which a carbon atom is connected to four different atoms or groups is said to be a chiral carbon or centre. Carbohydrates possessing one or more such carbon chiral centres are mostly optically active. Molecules rotating plane-polarized light to the left is laevorotatory (–) substance. [E.g., (-)-glucose]. Molecules, rotating plane-polarized light to the right are dextrorotatory (+) substance. E.g., (+)-glucose]. Most biologically important molecules have chiral centres, and hence are optically active. Often, living systems contain only one of the two forms, i.e. either (+) or (-). Molecules having more than one chiral centre, maybe (+) or (-) or optically active (meso) form. Tartaric acid exits in L(+), D(-) and meso tartaric forms. (+) or (-) is determined from the experimental determination of the optical activity of the molecule.