The Zeeman effect splits an atomic spectral line into several components in a magnetic field.
Pieter Zeeman discovered the effect in 1896 by observing changes in sodium spectral lines placed in a magnetic field. Hendrik Lorentz explained the simpler pattern using classical electron theory, and the phenomenon later became an important test of quantum models of angular momentum and magnetic moments.
A magnetic field changes the energies associated with different orientations of an atom’s angular momentum. When the atom emits or absorbs light, these slightly different energies produce multiple nearby frequencies rather than one line. The simple case is called the normal Zeeman effect; more complicated patterns are called the anomalous Zeeman effect and involve electron spin.
The Zeeman effect is distinct from the Stark effect, which splits or shifts spectral lines because of an electric field. Astronomers also use magnetic Zeeman signatures to study fields on stars and the Sun.