  The Gamma-Ray burst (GRB) - afterglow transition is one of the most
interesting and least studied GRB phases. During this phase the relativistic
ejecta begins interacting with the surrounding matter. A strong short lived
reverse shock propagates into the ejecta (provided that it is baryonic) while
the forward shock begins to shape the surrounding matter into a Blandford-McKee
profile. We suggest a parametrization of the early afterglow light curve and we
calculate (analytically and numerically) the observed parameters that results
from a reverse shock emission (in an interstellar medium [ISM] environment). We
present a new fingerprint of the reverse shock emission that is added to the
well known $t^{-2}$ optical decay. Observation of this signature would indicate
that the reverse shock dominates the emission during the early afterglow. The
existence of a reverse shock will in turn imply that the relativistic ejecta
contains a significant baryonic component. This signature would also imply that
the surrounding medium is an ISM. We further show that: (i) The reverse shock
optical flash depends strongly on initial conditions of the relativistic
ejecta. (ii) Previous calculations have generally overestimated the strength of
this optical flash. (iii) If the reverse shock dominates the optical flash then
detailed observations of the early afterglow light curve would possibly enable
us to determine the initial physical conditions within the relativistic ejecta
and specifically to estimate its Lorentz factor and its width.
