  We calculate the reverse shock (RS) synchrotron emission in the optical and
the radio wavelength bands from electron-positron pair enriched gamma-ray burst
ejecta with the goal of determining the pair content of GRBs using early time
observations. We take into account an extensive number of physical effects that
influence radiation from the reverse-shock heated GRB ejecta. We find that
optical/IR flux depends very weakly on the number of pairs in the ejecta, and
there is no unique signature of ejecta pair enrichment if observations are
confined to a single wavelength band. It may be possible to determine if the
number of pairs per proton in the ejecta is > 100 by using observations in
optical and radio bands; the ratio of flux in the optical and radio at the peak
of each respective reverse-shock light curve is dependent on the number of
pairs per proton. We also find that over a large parameter space, RS emission
is expected to be very weak; GRB 990123 seems to have been an exceptional burst
in that only a very small fraction of the parameter space produces optical
flashes this bright. Also, it is often the case that the optical flux from the
forward shock is brighter than the reverse shock flux at deceleration. This
could be another possible reason for the paucity of prompt optical flashes with
a rapidly declining light curve at early times as was seen in 990123 and
021211. Some of these results are a generalization of similar results reported
in Nakar & Piran (2004).
