[citation needed] This dependence of specific rotation on wavelength is called optical rotatory dispersion.
The first accounts in most cases for the majority of the variation in rotation and should not strictly be termed rotatory dispersion.
It depends on the fact that optical activity is actually circular birefringence.
In other words, a substance which is optically active transmits right circularly polarized light with a different velocity from left circularly polarized light.
In addition to this pseudodispersion which depends on the material thickness, there is a true rotatory dispersion which depends on the variation with wavelength of the indices of refraction for right and left circularly polarized light.
The two phenomena are closely related, just as are ordinary absorption and dispersion.
An object that cannot be superimposed on its mirror image is said to be chiral, and optical rotatory dispersion and circular dichroism are known as chiroptical properties.
Still other enzymes produce new chiral centers, always with a high specificity.
These properties account for the fact that optical rotatory dispersion and circular dichroism are widely used in organic and inorganic chemistry and in biochemistry.
In the absence of magnetic fields, only chiral substances exhibit optical rotatory dispersion and circular dichroism.
In a magnetic field, even substances that lack chirality rotate the plane of polarized light, as shown by Michael Faraday.
In regions of absorption, magnetic circular dichroism is observable.