Beyond the Thin Lens Approximation

We obtain analytic formulae for the null geodesics of Friedmann-Lema\^ıtre-Robertson-Walker spacetimes with scalar perturbations in the longitudinal gauge. We use these to provide a rigorous derivation of the cosmological lens equation. We obtain an expression for the magnification of a bundle of light rays in these spacetimes without restriction to static or thin lens scenarios. We show how the usual magnification matrix naturally emerges in the appropriate limits.