The dynamical Casimir effect in quantum electrodynamics, which occurs
for a frequency-modulated electromagnetic oscillator, is described in
terms of reflection backward in time of the oscillator mode. From an e
xperimental viewpoint, backward-in-time reflections appear as radiated
photons. The distribution of the electromagnetic radiation due to a m
odulation pulse is computed as a function of the frequency omega and i
s shown to exhbit a (1/omega) singularity in the limit omega --> 0.