We determine the complex refractive index (N) of a semiconducting CdSe thin film from the measurements of the transmittance spectrum. The experiment allow us to obtain, from the real part of refractive index, the N dispersive law, and from the imaginary part, the absortion coefficient. An energy dependence analysis of the absorption coefficient is done to obtain the value of the forbidden band ''GAP'' of thesemiconductor and the nature of the optical transition.