The frequency associated with the difference between two discrete energy levels in an atomic system , given by
where f 2,1 is the frequency associated with the difference between two energy levels, E 2 and E 1 ( E 2 > E 1 ), and is Planck's constant . Note: If a transition from E 2 to E 1 occurs, a photon with frequency f 2,1 is likely to be emitted. If the atomic system is at energy level E 1 , and a photon of frequency f 2,1 is absorbed, the energy level will be raised to E 2 . [ From Weik '89 ]