The resonant spectra of light reflected and transmitted by a heterostructure with a finite system of equidistant quantum wells have been calculated. Recurrence relations have been derived connecting the amplitude reflection coefficients from N and N - 2 quantum wells, and analytical properties of the reflection coefficient as a function of the complex frequency ω have been established. A method has been proposed which allows one to find complex frequencies of coupled exciton-photon nonstationary modes, or exciton-polaritons. It has been shown that the resonant Bragg structures represent a particular case where among N eigenmodes only one is radiative.