Semiclassical physics, or simply
semiclassical refers to a theory in which one part of a system is described
quantum-mechanically whereas the other is treated
classically. For example, external
fields will be constant, or when changing will be classically described. In general, it incorporates a development in
powers of
Planck's constant, resulting in the classical physics of power 0, and the first nontrivial approximation to the power of (-1). In this case, there is a clear link between the quantum-mechanical system and the associated semi-classical and classical approximations, as it is similar in appearance to the transition from
physical optics to
geometric optics.