This second paper on the Fabry–Perot cavity presents a semi-classical approach, which means that we
consider the transition from wave optics to geometrical optics. The basic concepts are the periodic
orbits and their stability. For the plano-concave Fabry–Perot cavity in the paraxial approximation,
the derivation of the trace formula demonstrates that the spectrum is based only on the axial
periodic orbit and its repetitions. Experiments with microlasers illustrate the relation to periodic
orbits. The methods presented in this paper are not limited to laser cavities and can be applied to
a large range of wave systems.