2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/116979General relativity includes geometrical optics. This basic fact has relevant consequences that concern the physical meaning of the discontinuity surfaces propagated in the gravitational field - as it was first emphasized by Levi-Civita.6 pages, LaTeX, to be published on Spacetime & SubstanceGeneral Physicstext