The strong similarities between the light propagation in a curved spacetime and that in a medium with graded refractive index are found.It is pointed out that a curved spacetime is equivalent to an inhomogeneous vacuum for fight propagation.The corresponding graded refractive index of the vacuum in a static spherically symmetrical gravitational field is derived.This result provides a simple and convenient way to analyse the gravitational lensing in astrophysics.
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