  In this paper we consider an initially excited two-level system coupled to a
monomode cavity, and compute exact expressions for the spectra spontaneously
emitted by each system in the general case where they have arbitrary linewidths
and frequencies. Our method is based on the fact that this problem has an
easily solvable classical counterpart, which provides a clear interpretation of
the evidenced phenomena. We show that if the cavity linewidth is much lower
than the atomic linewidth, photons are emitted at the cavity frequency, even if
the atom and the cavity are strongly detuned. We also study the links between
the spontaneous emission spectra and the fluorescence spectra emitted when the
atom is driven by a classical field of tunable frequency.
