2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/90693The Fermat principle indicates that light chooses the temporally shortest path. The action for this "motion" is the observed time, and it has no Lorentz invariance. In this paper we show how this action can be obtained from relativistic action, and how the classical wave equation of light can be obtained from this action.6 pages, no figures, Latex, discussion of eq. of motion and one appendix are addedQuantum Physicstext