To go into the domain of Cherenkov radiation, we now consider perpendicular distance b much greater than atomic distances in a medium, that is, | λ λ b | ≫ ≫ 1 . With this assumption we can expand the Bessel functions into their asymptotic form: E 1 ( ω ω ) → → i q ω ω c 2 ( 1 − − 1 β β 2 ε ε ( ω ω ) ) e − − λ λ b λ λ b