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 He demonstrated that
aqueous solutions of both types of crystals showed optical
rotation, equal in magnitude (for solution of equal concentration)
but opposite in direction