, clockwise direction, it
is called dextrorotatory (Greek for right rotating) or the d-form
and is indicated by placing a positive (+) sign before the degree
of rotation If the light is rotated towards left (anticlockwise
direction), the compound is said to be laevo-rotatory or the
l-form and a negative (–) sign is placed before the degree of
rotation Such (+) and (–) isomers of a compound are called
optical isomers and the phenomenon is termed as optical
isomerism (ii) Molecular asymmetry, chirality and enantiomers: The
observation of Louis Pasteur (1848) that crystals of certain
compounds exist in the form of mirror images laid the
foundation of modern stereochemistry