  The fluctuations observed in the light curves of some GRB-afterglows (such as
GRB 021004) provide a useful tool to probe the circum-burst density profile and
to probe the variations in the energy of blast-wave with time. We present a
general formalism that reduces the calculation of the observed light curve from
a Blandford-Mackee blast-wave to the evaluation of a one dimensional integral.
Using this formalism we obtain a simple approximation to the general light
curve that arises in more complex situations where the afterglow's energy or
the external density vary. The solution is valid for spherically symmetric
profiles and it takes a full consideration of the angular time delay effects.
We present the light curves of several external density profiles and
demonstrate the effects of density variations on the light curve. We also
re-visit the afterglow of GRB021004 and we find that the steep decay after the
first bump ($\sim 4000sec$) cannot result from a spherically symmetric density
variation or from the passage of the synchrotron frequency through the optical
band. This suggests that an angular structure is responsible to some of the
observed features in the light curve. This may be the first evidence that an
angular structure is important in the early stages of the afterglow.
