Reflection spectra of resonant reflectors have been solved on a computer. They can be divided into two types: degenerate and non-degenerate. Line shapes of resonant peaks, modulability of envelopes, characteristics of periodic spectra and two relevant properties are discussed in detail. The single axial mode operation in a tuning range of 2A has been obtained with a PZT tuned resonant reflector. This work provides a background for designing resonant reflectors.