Degree to which linearly polarized incident electromagnetic radiation becomes elliptically polarized in an absorbing optically active medium. \[\tan\;\psi = (E_{\rm{R}} - E_{\rm{L}})/(E_{\rm{R}} + E_{\rm{L}}) = \tanh (k^{\rm{L}} - k^{\rm{R}})\pi l/\lambda\] where \(E_{\rm{R}}\) and \(E_{\rm{L}}\) are the electric vectors of right and left circularly polarized radiation, \(k^{\rm{L}}\) and \(k^{\rm{R}}\) are the absorption indices of the sample for left and right circularly polarized radiation, \(\lambda\) is the wavelength of the radiation and \(l\) is the path-length through the medium.