# Computing gravitational deflection of light knowing $\phi$ and $-\nabla \phi$?

I have a 3D cartesian grid an in each grid I know the gravitational potential $\phi$ and the 3D gravitational field $-\nabla \phi$ (with a Newtonian approach). How to compute the path of a photon in these grids and its 3D deflection in each cell ?

Thank you very much.

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