2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/138531We obtain expressions for the Casimir energy and force following an approach which may be applied to cavities made up of arbitrary materials. In the case of planar cavities we obtain the well known Lifshitz formula. The approach is easily generalizable to other geometries.10 pages, three figures, to be published in Rev. Mex. FisQuantum Physicstext