What effect causes spectral lines to split in a magnetic field?

The Zeeman effect causes spectral lines to split in the presence of a magnetic field. The field interacts with the magnetic moments associated with electrons' orbital motion and spin, shifting atomic energy levels by different amounts.

Because an emitted or absorbed spectral line corresponds to an energy difference between atomic states, those shifted levels produce several nearby frequencies instead of one. The size of the separation depends on the magnetic-field strength, making the effect useful for measuring magnetic fields in laboratories, stars, and the Sun.

Dutch physicist Pieter Zeeman discovered the effect in 1896. Zeeman and Hendrik Lorentz received the 1902 Nobel Prize in Physics for explaining radiation phenomena in magnetic fields. The normal Zeeman effect produces a simple pattern, while the anomalous Zeeman effect involves electron spin and can produce more complicated splitting.

The main mix-up is with the Stark effect, which is caused by an electric field rather than a magnetic one. The Paschen–Back effect is not a separate basic answer here: it describes the strong-field limit of Zeeman splitting, when the magnetic field disrupts the atom's usual spin–orbit coupling.