Yeah, it's essentially a paraxial approximation. You can find derivations (through a few steps) by essentially doing the following reductions: Maxwell's -> Fou

Yeah, it's essentially a paraxial approximation. You can find derivations (through a few steps) by essentially doing the following reductions: Maxwell's -> Fourier optics (scalar wave phenomena) -> Eikonal equation (rays, but in inhomogeneous media) -> Ray optics (rays, but in almost-everywhere homogeneous media). I think most modern optics textbooks (see, e.g., Fundamentals of Photonics) cover the reductions and the corresponding steps.