The optical inactivity is due to two factors
(i) by external compensation
(ii) internal compensation.
External compensation occurs when the liquids are mixed externally and optically inactivity occurs. Internal compensation causes optical inactivity due to the configuration of molecules. They have plane of symmetry meso-tartaric acid is optically inactive due to internal compensation as its upper half rotates the plane polarised light towards clockwise direction and lower half towards anticlockwise direction. Hence, the rotation of upper half is compensated by lower half.
