  We present the study of parametric resonance in a one-dimensional cavity
based on the analysis of classical optical paths. The recursive formulas for
field energy are given. We separate the mechanism of particle production and
the resonance amplification of radiation. The production of photons is a purely
quantum effect described in terms of quantum anomalies in recursive formulas.
The resonance enhancement is a classical phenomenon of focusing and amplifying
beams of photons due to D\"{o}ppler effect.
