Bending of Light rays!

If light passes near a massive object, the gravitational interaction causes a bending of the ray. This can be thought of as happening due to a change in the effective refractive index of the medium given by $\large n(r) = 1 + 2\frac{GM}{rc^2}$ where $r$ is the distance of the point of consideration from the center of the mass of the massive body, $G$ is the universal gravitational constant, $M$ the mass of the body and $c$ the speed of light in vacuum. Considering a spherical object, find the deviation of the ray $\theta_0$ from the original path as it grazes the object.

If $\large \theta_0 = n_1\frac{GM}{rc^{n_2}}$ , find $n_1 + n_2$ .