A rotation in the plane of oscillation of a linearly polarized beam upon passage through a medium. It occurs in asymmetrical molecules where the electrons are more easily accelerated in one orientation than another. The random orientation of molecules in the medium does not cancel the effect, since flipped molecules retain the same handedness (just as a right-handed helix remains right-handed upon turning it upside down). Optical activity is much more prominent in nature than would be normally expected due to the fact that almost all amino acids (the building blocks of life) are left-handed.
The angle of rotation is defines as positive when the electric field is rotated clockwise,
where d is the thickness of the medium, is the vacuum wavelength, is the index of refraction for parallel polarization and is the index of refraction for perpendicular polarization. A medium in which is called d-rotary, and a medium in which is called l-rotary.
A quantity called specific rotary power is defined as
Faraday Rotation, Linear Polarization, Optical Rotation, Polarimetry, Specific Rotary Power
References
Yeh, P. and Gu, C. "Optical Activity and Faraday Rotation." §3.6 in Optics of Liquid Crystal Displays. New York: Wiley, pp. 75-86, 1999.
