  We show how to compute the magnetooptical functions (absorption, reflection,
and transmission) when Rydberg Exciton-Polaritons appear, including the effect
of the coherence between the electron-hole pair and the electromagnetic field,
and the polaritonic effect. Using the Real Density Matrix Approach the
analytical expressions for magnetooptical functions are obtained and numerical
calculations for Cu$_2$0 crystal are performed. The influence of the strength
of applied external magnetic field on the resonance displacement of excitonic
spectra is discussed. We report a good agreement with recently published
experimental data.
