The Zeeman Effect

Discovered by Dutch physicist Pieter Zeeman in 1896, the Zeeman effect was a pivotal observation in the development of modern physics. When an atom is placed in a magnetic field, its energy levels, which are normally degenerate (having the same energy), are split. This is because the magnetic field interacts with the magnetic moment of the atom. This magnetic moment has two primary sources: the motion of electrons orbiting the nucleus (orbital angular momentum) and the intrinsic quantum property of electrons known as spin (spin angular momentum). The interaction lifts the degeneracy of states with different magnetic quantum numbers.

When an electron transitions between these split energy levels, it emits or absorbs a photon. Since there are now multiple possible final energy states, a single spectral line, corresponding to a transition in the absence of a field, splits into several distinct lines. The spacing and polarization of these lines provide detailed information about the atom’s electronic structure, including its angular momentum quantum numbers. Hendrik Lorentz provided an early classical explanation, but a complete understanding required the advent of quantum mécanique and the concept of electron spin. The effect is categorized into the ‘normal’ Zeeman effect (for atoms with zero total spin) and the more common ‘anomalous’ Zeeman effect (for atoms with non-zero spin).