Zeemara

Meet Zeemara, the atom who stands in a magnet's gap and watches one of her spectral lines split into three.

An electron orbiting in an atom carries orbital angular momentum, and that makes it a tiny magnet. Its energy in an outside magnetic field depends on how that tiny magnet is oriented. Quantum mechanics allows only certain orientations, labelled by the magnetic quantum number m_ℓ. With no field, all orientations have the same energy. Switch on a field B and each one shifts by m_ℓ μ_B B, so a single level fans out into evenly spaced sublevels.

In the normal Zeeman effect, transitions can change m_ℓ only by −1, 0 or +1, so one spectral line becomes three. The spacing is set by the Bohr magneton, about 5.788×10⁻⁵ electronvolts per tesla, a tiny shift even in a strong laboratory magnet.

Astronomers use the same splitting to measure magnetic fields on the Sun: the strong fields in sunspots visibly split lines in their spectra.

An honest caveat: most atoms show the more complicated anomalous Zeeman effect, because electron spin adds its own magnetism. The neat three-line pattern is the simple case, and Zeemara is proud to be it.