Considering a plane wave the electron quantum parameter can be rewritten using this relation between electric and magnetic fields:where  is the wavevector of the plane wave and  the wavevector magnitude. Inserting this expression in the formula of :where the vectorial identity  was used. Elaborating the expression:Since  for a plane wave and the last two terms under the square root compensate each other,  reduces to:
In the simplified configuration of a plane wave impinging on the electron, higher values of the electron quantum parameter are obtained when the plane wave is counter-propagating with respect to the electron velocity.