The interest in unipolar pulses in optics is related to the possibility of their efficient action on quantum objects. In this work, we study the interaction of a short rectangular unipolar pulse with a two-level resonant medium under conditions that the pulse duration is shorter than the relaxation times of the medium. It is paradoxical that the polarization and the population difference of the medium oscillate not at the Rabi frequency, but, rather, at a frequency that is proportional to the sum of the squared frequency of the resonant transition of the medium and the squared Rabi frequency of the pulse. This phenomenon is similar to the Stark effect, which consists of a shift of the transition frequencies of a medium in a constant external electric field. The possibility of generating radiation at frequencies that exceed the resonant transition frequency of the medium upon its excitation by a rectangular unipolar pulse is discussed.