Chiral compounds have a unique property to rotate the plane polarised light in either clockwise direction or anti clockwise direction.This is called as the optical activity of the chiral compounds. Plane polarised light can travel only in one plane.
The rotation is fixed for the pure substances. The rotation of the plane polarised light can be used to identify the substance, to find the purity of the substance and to know the concentration. Sugar concentration is measure in a same way in industries. There is a fixed optical rotation for the sugar by which they can measure the concentration of the sugar in the given sample.