In atomic physics, the fine structure describes the splitting of the spectral lines of atoms.
The gross structure of line spectra is due to the principal quantum number n, giving the main electron shells of atoms. However, on closer examination, electron shells with n > 1 exhibit fine structure, and lines are split due to spin-orbit coupling (the energy difference between the electron spin being parallel or antiparallel to the electron's orbital moment). This gives rise to for example the doublet in the yellow sodium D-line. The fine structure of hydrogen is actually two separate corrections to the Bohr energies: one due to the relativistic motion of the electron, and the other due to spin-orbit coupling; see Lamb shift. Fine level structure can be split further by interaction with the magnetic moment of the nucleus (hyperfine structure).

Spin-orbit coupling

Spin-orbit interaction
Angular momentum coupling