This work is an attempt to review some of the most famous and important studies to correlate the optical-linear refractive index of a substance to its energy band-gap.The refractive index of different types of materials, like semiconductors, insulators, oxides, thin-films, and oxide glasses, has been reported in a large number of publications in attempts to the estimation of how it can be correlated to the optical band-gap. The present work can be considered as a comparative study between the most successful relations in correlating the refractive index to the energy gap. The careful reviewing of the previous studies to correlate the optical refractive index and energy gap led to concluded that Reddy and Ahmmed approximation was selected to develop a new realized form that can give a good fit to the experimental data and, hence, be used directly as an accurate formula in the theoretical studies. The obtained formula correlates the optical refractive index to the energy gap and the oxygen atom's electronegativity. Such relation has been used to calculate the refractive indices for more than 96 materials (elements/compounds) with a high agreement with the experimental data.