Optical rotation of a substance is the angle through which the change or rotation in plane of polarisation occurs when polarised  light passes through optically active substances.

Rotation of oreientation of linearly polarised was observed in early 1800’s by jean baptiste biot.

Since in any given solution containing the a chiral compound we have billions of molecules all in different orientaions, it is statistically certain that there will be a cancellation of rotations with the net result that as the light exits the sample the light will have no apparent net rotation. Such a chiral molecules are said to be optically inactive. 
the rotation of the plane polarized light by one configuration is not canceled out or negated by rotation of the light by the opposite configured molecule because it is not present. Therefore there will be a net rotation observed as the light passes throgh the sample. Such a compound is said to be optically active. 


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