Molecular asymmetry, arising from the non-superimposability of mirror images due to an asymmetric carbon, is responsible for the optical activity observed in organic compounds, where the mirror images rotate plane-polarized light in equal magnitude but opposite direction.

In 1874, J. Van’t Hoff and C. Le Bel independently proposed that if all substituents around a central carbon are different, the resulting molecule is non-superimposable with its mirror image, leading to the concept of an asymmetric carbon or stereocentre ...

Louis Pasteur observed that crystals of certain compounds exist in the form of mirror images, and solutions of these crystals showed equal-magnitude but opposite-direction optical rotation, leading to the concept of molecular asymmetry.

The phenomenon of having (+) and (–) isomers in optical isomerism is termed as optical isomerism, where compounds exhibit different rotations of plane-polarized light, leading to dextrorotatory and laevo-rotatory isomers.

Dextrorotatory isomers are denoted by placing a positive (+) sign before the degree of rotation, indicating a clockwise rotation. Laevo-rotatory isomers are denoted by a negative (–) sign before the degree of rotation, indicating a counterclockwise rotation.