Illustrated Glossary of Organic Chemistry

Specific rotation ([α]): The measure of a substance's optical activity, normalized for concentration of the sample.

[α] = the specific rotation in degrees of specified wavelength of light and specified temperature
t = temperature of measurement
λ = wavelength of light; usually the D line from a sodium vapor lamp (589 nm)
α = observed rotation in degrees
l = path length (length of sample tube) in decimeters (one decimeter = 10 cm)
c = concentration of sample measured in grams per milliliter of solution