  We study the impact of a spatially homogeneous yet non-stationary dielectric
permittivity on the dynamical and spectral properties of light. Our choice of
potential is motivated by the interest in PT-symmetric systems as an extension
of quantum mechanics. Because we consider a homogeneous and non-stationary
medium, PT symmetry reduces to time-reversal symmetry in the presence of
balanced gain and loss. We construct the instantaneous amplitude and angular
frequency of waves within the framework of Maxwell's equations and demonstrate
the modulation of light amplification and attenuation associated with the
well-defined temporal domains of gain and loss, respectively. Moreover, we
predict the splitting of extrema of the angular frequency modulation and
demonstrate the associated shrinkage of the modulation period. Our theory can
be extended for investigating similar time-dependent effects with matter and
acoustic waves in PT-symmetric structures.
