  We discuss the optical and radio early afterglow emission of the reverse
shock that crosses a baryonic ejecta as it interacts with the external
interstellar medium (ISM). We show that the peak of the optical flash divides
the light curve of the reverse shock into two distinctive phases. The emission
after the peak depends weakly on the initial conditions of the ejecta and
therefore it can be used as an identifiable signature of a reverse shock
emission. On the other hand, the emission before the optical peak is highly
sensitive to the initial conditions and therefore can be used to investigate
the initial hydrodynamic profile of the ejecta. In particular, if the prompt
$\gamma$-ray emission results from internal shocks, the early reverse shock
emission should resemble a smoothed version of the prompt $\gamma$-ray light
curve.
