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, E2 and E1 (E2 > E1,) and is Planck's constant. Note: If a transition from E2 to E1 occurs, a photon with frequency f 2,1 is likely to be emitted. If the atomic system is at energy level E1, and a photon of frequency f 2,1 is absorbed, the energy level will be raised to E2.