Optical Rotation animation   http://ja01.chem.buffalo.edu/~jochena/research/opticalactivity.html

Optical rotation
Two phenomena play an important role in the context of optical activity. One of them is optical rotation.
The graphic below shows an animation of a linear polarized light wave as it passes through an optically active medium (indicated by the rectangular box --not to scale). 

Optical rotation means the plane of polarization of a linearly polarized light beam rotates as it passes through an optically active medium, for instance a solution of chiral molecules. The rotation angle is proportional to the path length through the medium, and in case of a solution also to the concentration of the chiral substance. (See equation for relationship between observed and specific rotation.)

Optical rotation is measured by polarimetry. The wavelength used in polarimetry experiments is often 589.3 nm (this is the yellow light from a sodium lamp). The length of the cuvette is usually 10 cm.

Click on the Picture box below to see the animation.

Identically prepared solutions of the two enantiomers of a chiral molecule rotate the polarization plane in equal but opposite directions. A mixture of equal amounts of the two enantiomers (racemate) or a non-chiral substance does not exhibit this effect.

However, the sign of the rotation is not in a simple way related to the chiral structure in the sense that, say, an R configuration would always have a positive or negative optical rotation. Therefore, without additional structural information it is not possible to deduce an unknown absolute configuration of a molecule simply from the sign of the optical rotation. Computations are quite useful in this case.