Fine structure
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In atomic physics, the fine structure describes the splitting of the spectral lines of atoms.
The gross structure of line spectra is is the number of lines and their placement. This is determined by the differences in the energy levels of the various atomic orbitals. However, on closer examination, each line exhibits a detailed fine structure. This structure is due to small interactions that give small shifts and splittings of the energy levels. They may be analyzed by means of perturbation theory.
The spin-orbit interaction
Classically, orbiting charges possess a magnetic dipole moment, and this holds in quantum mechanics also. Slightly more surprisingly, the intrinsic angular momentum of particles due to spin gives them a magnetic moment; thus the 'spin-half' electron acts like a little magnet. Since it is orbiting the positively charged nucleus, it 'sees' a current, and hence a magnetic field.Template:Physics-stub