When circularly polarized light passes through an absorbing optically active medium, the speeds between right and left polarizations differ ( c L ≠ ≠ c R ) as well as their wavelength ( λ λ L ≠ ≠ λ λ R ) and the extent to which they are absorbed ( ε ε L ≠ ≠ ε ε R ). Circular dichroism is the difference Δ Δ ε ε ≡ ≡ ε ε L − − ε ε R . The electric field of a light beam causes a linear displacement of charge when interacting with a molecule (electric dipole), whereas its magnetic field causes a circulation of charge (magnetic dipole). These two motions combined cause an excitation of an electron in a helical motion, which includes translation and rotation and their associated operators. The experimentally determined relationship between the rotational strength R of a sample and the Δ Δ ε ε is given by