In this task you will be given a text passage from a trivia quiz. You should choose the category this question belongs to from these categories: History, Science, Social Science, Fine Arts, Literature. You can only choose one category, so if in doubt, please choose the closest match.

For particles with j=1/2 that undergo this effect, their Hamiltonian may be solved exactly to give the Breit-Rabi formula. In the strong-field limit of this effect, fine structure can be ignored and it is known as the Paschen-Back effect. The anomalous form of this effect was first observed in the d-lines of sodium, and it occurs when the total spin of the electrons involved is not zero. For 10 points, name this process where spectral lines are split by an external magnetic field, which is contrasted with the Stark effect.
Science