Zeeman Effect

Atoms emit or absorb light when electrons transition between discrete energy levels. In a magnetic field, the magnetic interaction changes the energies of states with different magnetic orientations. A transition that would produce one spectral frequency without the field may therefore produce several nearby frequencies. The observed pattern depends on the atomic levels involved, the field strength, and the geometry and polarization of the observed light. In weak fields, magnetic sublevels can often be described using the angular-momentum quantum number J and its projection M; their first-order energy shifts are proportional to the field and to a level-dependent factor called the Landé g factor.

The simplest, normal Zeeman pattern consists of three components under suitable conditions. Real atomic spectra often show more intricate, anomalous patterns, with a different number and spacing of components because electron spin and the coupling of orbital and spin angular momentum affect the levels. The splitting is generally small compared with the original optical frequency, so it is resolved with suitable spectroscopic instruments. Interpreting the pattern can reveal information about the quantum states participating in a transition.

Dutch physicist Pieter Zeeman reported the effect in 1896 after observing changes in sodium spectral lines in a magnetic field. The broadening he initially saw was later resolved as a splitting into distinct components. Hendrik Antoon Lorentz had developed a theoretical account connecting magnetism and light, and the two physicists shared the 1902 Nobel Prize in Physics for their work on the effect of magnetism on radiation.

In laboratory spectroscopy, Zeeman patterns help identify and characterize atomic energy levels and their magnetic properties. In astronomy, measuring the polarization and splitting of spectral lines provides a way to infer magnetic fields in the Sun and other stars. Such measurements require models that account for the relevant atomic transitions, field regime, line broadening and observing geometry; the spectrum is not simply a universal one-line-per-field indicator.