Zeeman effect

The Zeeman effect is the splitting of an atom's spectral lines into several closely spaced lines when the atom sits in a magnetic field. Without the field, the different orientations of an electron's angular momentum — the 2ℓ+1 values of m — all share the same energy, so they produce a single line. The field breaks that tie, nudging each orientation to a slightly different energy and fanning one line into a small family.

The cause is direct. Angular momentum carries a magnetic moment, like a tiny bar magnet, and a magnet has lower energy when aligned with a field and higher when opposed. Since the orientations are quantized, so are these energy shifts, each proportional to m and to the field strength. Counting the resulting lines is, in effect, counting the allowed values of m — a beautifully concrete window onto space quantization.

Discovered by Pieter Zeeman in 1896, the effect became one of the sharpest early probes of atomic structure and remains a workhorse today, from measuring the magnetic fields of distant stars and sunspots to controlling atoms in the laboratory. The full story also revealed the so-called anomalous Zeeman effect, whose extra splitting could only be explained once electron spin was added to the picture.