Page "Spectral line" Paragraph 5

Spectral lines are the result of interaction between a quantum system ( usually atoms, but sometimes molecules or atomic nuclei ) and a single photon.

When a photon has about the right amount of energy to allow a change in the energy state of the system ( in the case of an atom this is usually an electron changing orbitals ), the photon is absorbed.

Then it will be spontaneously re-emitted, either in the same frequency as the original or in a cascade, where the sum of the energies of the photons emitted will be equal to the energy of the one absorbed ( assuming the system returns to its original state ).

The direction and polarization of the new photons will, in general, correlate with those of the original photon.