Zeeman Effect
In 1896, Pieter Zeeman discovered that spectral lines split when atoms are placed in a magnetic field. Each electron energy level with angular momentum quantum number L splits into 2L+1 sublevels. Selection rules (Δm = 0, ±1) produce distinct polarization patterns visible perpendicular or parallel to the field.
Quantum mechanics of the Zeeman effect
An electron in a magnetic field B has energy shifted by ΔE = m_J · g_J · μ_B · B, where m_J is the magnetic quantum number, g_J is the Landé g-factor, and μ_B = 9.274×10⁻²⁴ J/T is the Bohr magneton.
The normal Zeeman effect (g_J = 1, spin zero) produces exactly 3 lines: a central π component (Δm = 0) and two σ components (Δm = ±1). The anomalous Zeeman effect arises when spin is nonzero and g_J ≠ 1, producing complex multiplets.
Viewing transverse to B: all three polarizations visible (π is linearly polarized along B; σ± are linearly polarized perpendicular). Viewing along B: only σ± visible, circularly polarized — the π line vanishes by selection rules.