Abstract <p>This study presents a detailed comparative analysis of the structural and optical properties of cadmium sulfide (CdS) in zinc-blende and wurtzite phases. The density functional theory (DFT) within the framework of pseudo-potentials and plane-waves are used. Calculations were performed using the Cambridge Serial Total Energy Package (CASTEP) code and the generalized gradient approximation (GGA). Key properties, including lattice parameters, refractive index, dielectric function, optical conductivity, absorption coefficient, and electron energy losses, are systematically computed and analyzed. The originality of this work lies in its broad absorption spectrum and the in-depth study of essential optical properties such as excitonic transitions and plasmonic resonances. These findings shed new light on the suitability of CdS for optoelectronic and photovoltaic applications, and highlight its potential for cutting-edge technologies in these fields.</p>