We develop a quantum mechanical model for the interaction of excitons in a microcavity embedded quantum well with a short resonant laser pulse. The model assumes full in-plane coherence. A Gaussian distribution of exciton slates is included in order to take into account the exciton inhomogeneous broadening. A coupling with a classical electromagnetic held provides the initial excitation pulse, and the time dependent emitted signal is calculated. Very good agreement with the recent results of time-resolved light emission under femtosecond resonant excitation by Norris et al. is obtained. We show that, in the strong coupling regime, the Rabi oscillations provide an energy transfer mechanism between the inhomogeneously broadened exciton levels, not present in the weak coupling regime or in the case of bare quantum;wells, which explains the very fast decay time of the measured signal. This mechanism is considerably faster than the scattering of excitons due to interface roughness. Consequently, the in-plane wave vector is conserved in the polariton dynamics.