2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/85416The physical nature of numerous of the nonlinear phenomena in optics is explained by inequality of forward and reversed optical transitions, that corresponds to a principle of time invariance violation in electromagnetic interactions. The direct and indirect experimental proofs of such inequality and direction of the further researches are discussed.11 pages LaTeX2e, 4 PS figuresOpticsGeneral Physicstext