text: For an incident unpolarized photon of energy , the differential cross section is:
  where
  *  is the classical electron radius (~2.82 fm,  is about 7.94 × 10 m or 79.4 mb)
  *  is the ratio of the wavelengths of the incident and scattered photons
  *  is the scattering angle (0 for an undeflected photon).
  The angular dependent photon wavelength (or energy, or frequency) ratio is
  as required by the conservation of relativistic energy and momentum (see Compton scattering). The dimensionless quantity  expresses the energy of the incident photon in terms of the electron rest energy (~511 keV), and may also be expressed as  increases monotonically with the deflection angle, from  (forward scattering, no energy transfer) to  (180 degree backscatter, maximum energy transfer). 
  In some cases it is convenient to express the classical electron radius in terms of the Compton wavelength: , where  is the fine structure constant (~1/137) and  is the reduced Compton wavelength of the electron (~0.386 pm), so that the constant in the cross section may be given as: