If an atom is already in the excited state, it may be agitated by the passage of a photon that has a frequency ν 21 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 ν 21. 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.