An electron drops from the fourth energy level, in an atom to the third level and then to the first level. Two frequencies of light are emitted. How does their combined energy compare with the energy of the single frequency that would be emitted if the electron dropped from the fourth level directly to the first level?
The combined energies of the two-photon process equals that of the single-photon process.
(E4 - E3) + (E3 - E1) = E4 - E1
1st photon 2nd photon single photon