Explain the following terms :
Optical activity

Explanation

Optical activity : When a plane polarised light is passed through a solutions of certain compounds, the light is rotated either in clockwise or in a anti - clockwise direction by a certain angle. This property of a compound of rotating plane polarised light from its path is known as optical activity.
If the compound rotates plane of polarised light in a 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), the compound is said to laevo-rotatory or the l - form and a negative (-) sign is placed before the degree of rotation. Such (+) and (-) isomers of compound are called optical isomers and the phenomenon is termed as optical isomerism.
Louis Pasteur believed that the difference in optical activity is due to difference in 3D arrangement of atoms in a molecule.