 Circular dichroism (CD) is then defined by the difference between left (formula_14) and right (formula_15) circularly polarized light, formula_16, following the sign convention of natural optical activity.In the presence of a static, uniform external magnetic field applied parallel to the direction of propagation of light, the Hamiltonian for the absorbing center takes the form formula_17 for formula_18 describing the system in the external magnetic field and formula_19 describing the applied electromagnetic radiation.The absorption coefficient for a transition between two eigenstates of formula_18, formula_21 and formula_22, can be described using the electric dipole transition operator formula_23 as 

The formula_26 term is a frequency-independent correction factor allowing for the effect of the medium on the light wave electric field, composed of the permittivity formula_27 and the real refractive index formula_28.