If a spectral lamp is placed in a magnetic field and examined with a spectral apparatus, a widening of the spectral lines is observed as the magnetic field increases. If the resolution of the spectral apparatus and the magnetic field are large enough, the splitting of the spectral lines into several components can be observed.
This effect is named after the Dutchman Pieter Zeeman, who made this observation on the sodium D lines in 1896. Already in 1862 Michael Faraday had noticed a widening of the magnetic field on the same lines.
Zeeman's observations showed that the energy levels of atoms are modified by an external magnetic field in such a way that, at radiative transitions between such levels, spectral components with higher and lower frequencies than those of the original line also occur and that the size of the splitting is directly proportional to the magnetic field strength if the magnetic field does not become too strong. The individual components also have a defined polarization, which is only dependent on the direction of observation relative to the magnetic field. Components observed in the field direction have circular polarization, perpendicular to the field direction linear polarization.