<p>This study investigates the size- and dimensionality-dependent optical properties of PdSe₂ and InSb nanostructures using a cohesive-energy-based model combined with band-gap, refractive-index, and reflectivity relations. Three structural regimes were considered: 0D quantum dots, 1D nanowires/nanotubes, and 2D layered nanosheets. The results show that the band gap decreases with increasing particle size, nanowire diameter, or layer number, reflecting the weakening of quantum confinement and the transition toward bulk-like behavior. In contrast, the refractive index and reflectivity increase with structural size because of the inverse relationship between band gap and optical polarizability. PdSe₂ generally exhibits larger band-gap values, whereas InSb shows relatively higher refractive-index and reflectivity values in larger structures due to its narrow bulk band gap. The model predictions show reasonable agreement with available experimental and theoretical data, particularly for band-gap trends, while deviations in refractive index and reflectivity are attributed to the empirical nature of the Moss and Fresnel relations and the absence of wavelength-dependent, anisotropic, and surface-related effects. The results demonstrate that the cohesive-energy approach can provide a useful comparative framework for describing dimensionality-dependent optical trends in semiconductor nanostructures.</p>