Geometric optics limit is known to be a good-enough approximation for the calculation of distances and image distortions in the Universe. In cosmology, we usually assume a point source that emits spherical waves and only a small section of the wavefront is accessible to the observer. This instantaneous wavefront is represented by a thin bundle of rays. In this talk, I will outline the analogies between the paraxial ray optics of the Newtonian theory and thin bundles in general relativity. I then propose a method adopted from the paraxial wave optics in order to study coarse-grained, ''wave-like'' effects of light beam propagation. In this method, the idea is to use certain phase space methods and symplectic symmetries to superpose two bundles initiated from a small yet finite sized source. With this, we explore the possibility of obtaining inhomogeneous intensity profiles on the transverse plane associated with the fundamental Gaussian mode. We observe that the form of such Gaussian beams is preserved throughout the propagation in curved spacetime on account of the symplectic symmetries of the underlying phase space. Finally, we show that the caustics can be avoided with this method which has potential effects on cosmological distance estimation.