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Atomic physics studies the structure and dynamics of atoms, particularly their electrons and their interactions with radiation and external fields. The atomic nucleus supplies charge and mass, but changes within the nucleus belong primarily to nuclear physics.

The hydrogen atom is a useful starting point because one electron moves in an approximately Coulombic potential. Its discrete energy differences produce spectral lines. More complex atoms add electron interactions, angular-momentum coupling and field-induced shifts. Spectroscopy tests those models by comparing predicted and observed transition frequencies.

An atomic physicist who compiles detailed reference data for alkali chemical atoms, including rubidium-87. His tables and derivations connect measured transition frequencies, hyperfine structure, Zeeman shifts and transition strengths to calculations used in laser cooling and precision spectroscopy. For example, the Breit-Rabi formula and the rubidium-87 clock transition data let an experimenter estimate how a residual magnetic field shifts a frequency reference.^[ref]

The transition frequency between two stationary energy levels is the positive frequency satisfying $h\nu =|E_f-E_i|$. In absorption, the system gains this energy from the radiation; in emission, it loses it. The corresponding angular frequency is $\omega =2\pi \nu$.

An energy difference specifies where a spectral line can occur, but does not by itself guarantee that the transition is strong or allowed. The interaction and its selection rules determine the transition amplitude. External fields, motion and finite lifetimes can shift or broaden the measured line.