Observed rotation of an optically active compound, measured using the polarimeter, depends on the experimental conditions and, therefore, is not a characteristic property of the compound. Specific rotation (symbol: [α]λT) of an optically active compound is defined as follows:
α = observed rotation measured using a polarimeter
l = length of sample tube
C = concentration, if a solution of the compound is used for the experiment, or density, if a neat sample of the compound is used
T = Temperature (usually 25 ºC)
λ = wave length of the light used (usually 589 nm)
Specific rotation of a compound is a characteristic property of the compound as long as the temperature, the wave length of the light, and, if a solution is used for the experiment, the solvent are specified. The units of specific rotation are degreesmLg-1dm-1. However, since the units of specific rotation are always the above, traditionally, specific rotation is reported without units.