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The purpose of the investigation is to study the interaction between an obliquely incident, plane electromagnetic wave and a plane, semi infinite dielectric medium, the dielectric constant of which contains a space- and time- varying component. Let us first consider a medium, the dielectric constant of which is periodically fluctuating only in time. It can then be shown that one gets parametric resonance when the angular frequency, w1, of the incident electro-magnetic wave is equal to half the angular frequency, w2, of the forced, or pumped, variations in the disturbed medium. However, when the disturbance is propagating, this resonance condition becomes altered. We find, that in order to get resonance effects certain relations depending on the angle of incidence and the phase velocities of the incident and pump-wave, predicted by Rydbeck, must be fulfilled. The harmonic time variation of the dielectric medium produces transmitted and reflected waves (non-linear modes) on the characteristic frequencies w1 = nw2, where n 0, 1, 2 ... The angles of reflection and transmission will be functions of the propagation parameters of the two waves as expressed by a generalization of Snell's law. The investigation, finally, also includes the case where losses are taken into account.