If an atom is already in the excited state, it may be agitated by the passage of a photon that has a frequency ν21 corresponding to the energy gap Δ"E" of the excited state to ground state transition. In this case, the excited atom relaxes to the ground state, and it produces a second photon of frequency ν21. The original photon is not absorbed by the atom, and so the result is two photons of the same frequency. This process is known as "stimulated emission".