The Zeeman effect is an effect of a magnetic field on spectral lines: electron orbitals that would normally be degenerate (identical binding energy) are influenced by presence of a magnetic field so that they differ. The energy associated with each electron orbital is influenced by magnetism: it could be looked at as the influence of magnetic force on the orbiting electrons, but that seems a classical view which may not be the real picture. The Zeeman effect also affects the polarization of the photons. The effect is observable with a spectropolarimeter, a useful instrument for clues to astronomical magnetic fields. Zeeman-Doppler imaging (ZDI) is a technique for using the effect.