  Reduced electron screening in two-dimension plays a fundamental role in
determining exciton properties, which dictates optoelectronic and photonic
device performances. Considering the explicit electron-hole interaction within
the $GW-$Bethe-Salpeter formalism, we first study the excitonic properties of
pristine phosphorene and investigate the effects of strain and impurity
coverage. The calculations reveal strongly bound exciton in these systems with
anisotropic spatial delocalization. Further, we present a simplified hydrogenic
model with anisotropic exciton mass and effective electron screening as
parameters, and the corresponding results are in excellent agreement with the
present $GW-$BSE calculations. The simplified model is then used to investigate
exciton renormalization in few-layer and heterostructure phosphorene. The
changes in carrier effective mass along with increasing electron screening
renormalizes the exciton binding in these systems. We establish that the
present model, where the parameters are calculated within computationally less
expensive first-principles calculations, can predict exciton properties with
excellent accuracy for larger two-dimensional systems, where the many-body
$GW-$BSE calculations are impossible.
