  We found strong similarities between the gravitational lensing and the
conventional optical lensing. The similarities imply a graded refractive index
description of the light deflection in gravitational field. We got a general
approach to this refractive index in a static spherically symmetric
gravitational field and obtained its exterior and interior solutions exactly
through the general relativity. In weak field case, the two solutions come to a
simple unified exponential function of the gravitational potential. With these
results, the gravitational lensing can be analyzed in a convenient optical way.
Especially, the long puzzling problem of the central image missing can be
solved easily. We also pointed out that the graded refraction property of the
gravitational spacetime is related to the vacuum influenced by the
gravitational matter.
