  Single photon, emitted in a transition between two states, has a frequency
distribution of intensity, which is given by Lorentzian if the transition is
only naturally broadened and the period of observation $T$ is long compared to
the lifetime $T_{1}$ of the excited state. However, when the observation time
$T$ is short or comparable to $T_{1}$, the frequency spectrum is appreciably
broadened. If only the delayed part of the emitted radiation field is detected,
then the radiation spectrum does not change. However, if the radiation field is
transmitted through a resonant absorber and then detected, the
absorption/transmission spectrum of the delayed radiation field is narrowed. We
show that this narrowing is due to the interference of the spectral components
of the field. Experimental spectra of absorption of M\"{o}ssbauer radiation,
obtained by coincidence technique, confirm this conclusion.
