Gravitational Microlensing I: A Unique Astrophysical Tool
  In this article we review the astrophysical application of gravitational
microlensing. After introducing the history of gravitational lensing, we
present the key equations and concept of microlensing. The most frequent
microlensing events are single-lens events and historically it has been used
for searching dark matter in the form of compact astrophysical halo objects in
the Galactic halo. We discuss about the degeneracy problem in the parameters of
lens and perturbation effects that can partially break the degeneracy between
the lens parameters. The rest of paper is about the astrophysical applications
of microlensing. One of the important applications is in the stellar physics by
probing the surface of source stars in the high magnification microlensing
events. The astrometric and polarimetric observations will be complimentary for
probing the atmosphere and stellar spots on the surface of source stars.
Finally we discuss about the future projects as space based telescopes for
parallax and astrometry observations of microlensing events. With this project,
we would expect to produce a complete stellar and remnant mass function and
study the structure of Galaxy in term of distribution of stars along our line
of sight towards the centre of galaxy.
