The term specific rotation is used to compare the rotational effect of all optically active substances.
Specific rotation for a given wavelength of light at a given temperature is defined as the rotation produced by one-decimeter length of the liquid column containing 1 gram of the active material in 1cc of the solution.
If θ is the angle of rotation produced by l decimeter length of a solution of concentration C in gram per cc, then the specific rotation S at a given wavelength λ for a given temperature t is given by
The instrument used to determine the optical rotation produced by a substance is called polarimeter.
Sugar is the most common optically active substance and this optical activity is used for the estimation of its strength in a solution by measuring the rotation of plane of polarisation.