optical isomerism

Optical isomers 

Those isomers which have same molecular formula and same functional group (i.e., the connectivity between atoms is the same), but different spatial arrangements of the atoms, and have non-superimposable mirror images. Each non-superimposable mirror image structure is called an enantiomer.

Optical Activity 

Optical isomers can occur when there is an asymmetric carbon atom. An asymmetric carbon atom is one which is bonded to four different groups. It forms a chiral centre of the molecule. The four groups can be something hideously complex, or something comfortably simple like a hydrogen atom or a chlorine atom. Optical activity is the ability of a chiral molecule to rotate the plane of plane-polairsed light, measured using a polarimeter.  A simple polarimeter consists of a light source, polarising lens, sample tube and analysing lens. A solution of chiral compounds - light emerges with its plane of polarization changed - the solution is optically active and rotates the plane of polarized light clockwise or counterclockwise.

Dextrorotatory (+) compounds rotate plane polarized light clockwise Latin - dextro - “to the right”(d).

Levorotatory (-) compounds rotate plane polarized light counter clockwise Latin - levo - “to the left”(l).