In this study, the optical properties of bulk and thin films of VO2, V2O3, and V2O5, deposited on Al2O3 substrates, have been analyzed from infrared to vacuum ultraviolet range (up to 12 eV). Utilizing the available data of wavelength dependent optical constants of these materials in the literature, the energy corresponding to the peaks in the imaginary part of the dielectric function (epsilon (2)-R spectra), have been interpreted and compared as a function of structure, polarization, and temperature. The energies corresponding to the peaks in reflectivity-energy (R-E) spectra are explained in terms of the Penn gap (E (p)). E (p) values for VO2 and V2O5 are close to the average of energies corresponding to the peaks () while, their values are even closer in V2O3, reflecting the degree of anisotropy in the order of V2O3 < VO2 < V2O5. The first order reversible, insulator to metal phase transition (IMT) of both bulk and thin films of the V-O systems are studied as an effect of temperature change. The effective number of electrons, n (eff), participating in the optical transitions is described from the numerical integration using the well-known sum rule. The change in n (eff) with respect to the energy of incident photons is also calculated and it is found that this change is consistent with the peaks observed in the epsilon (2)-E spectra.