The Zeeman effect describes the change and splitting of an atom’s energy levels when an external magnetic field is applied.
A magnetic field interacts with the magnetic moments associated with an atom’s orbital and spin angular momentum. Energy levels that were previously degenerate can separate into distinct levels, producing multiple spectral lines instead of one. The pattern depends on the atom, the field strength, and the relevant quantum states.
Pieter Zeeman discovered the effect in 1896 by observing broadened or split spectral lines. Hendrik Lorentz provided a classical explanation, and Zeeman and Lorentz shared the 1902 Nobel Prize in Physics. Quantum mechanics later explained the more complex anomalous Zeeman effect, in which electron spin plays an important role.
The Zeeman effect is often confused with the Stark effect. Both involve splitting of atomic energy levels, but the Zeeman effect is caused by a magnetic field, whereas the Stark effect is caused by an electric field. Astronomers use Zeeman splitting to measure magnetic fields in stars and other astronomical objects.