2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/84096The starting point of the article is the puzzling fact that one cannot recover the Fresnel coefficients by letting tend the width of a slab to infinity. Without using the so-called limiting absorption principle, we show by a convenient limit analysis that it is possible to define rigorously the field diffracted by a semi-infinite periodic medium.14 pages, 13 figures, submitted to Optics CommunicationsOpticsComputational Physicstext