We have already learned from Experiment 1, how chiral molecules are optically active and how chiral substances can be characterized on the basis of their specific rotation [α]. You will have noticed that in that experiment we have used a sodium lamp with a wavelength of 589 nm as the light source, and may be wondering why the wave length of light source needed to be specified.

Well, as has been explained in the following video using sucrose solution as an example, the specific rotation [α] for a given substance, and hence the molar rotation [φ], varies as a function of the wavelength (λ) of the light source.This variation in the optical rotation ([α] or [φ] ) of a substance, with the variation in the wavelength (λ) of light, is known as optical rotatory dispersion (ORD).

When a plane polarised wave passes through an optically active medium some times its state of polarisation also gets affected along with its plane of polarisation and it comes out from the medium as an elliptically polarised light. This phenomena is known as Circular Dichroism.