When an atom becomes excited it can emit light at discrete frequencies determined by the energy levels available to it. For example, if the electron in a hydrogen atom is excited to the 6s orbital, it can give off light by jumping down to the 5s, 4p, or 3d orbitals. This atomic absorption spectrum is similar to a fingerprint as it is unique to every atom. By taking this spectrum for an unknown sample, we can determine whether a particular atom is present.