Polarization knowledge is important to detect object characteristics. An approximate relationship between vertically and horizontally polarized reflectivities of specular surfaces is developed and validated using the refractive index datasets of water in the various wavelength ranges at various incidence angles. This study proposes a unique technique to estimate the refractive indexes of a specular surface at a given view angle by the direct inversion of the Fresnel equation and the decomposition of the unpolarized emissivity. The unpolarized emissivity is calculated using the Fresnel equation and the refractive index of water at wavelengths ranging from ultraviolet (200 nm) to microwave (18.75 cm). Consequently, the differences of reflectivity between the Fresnel equations and the Hong approximation are approximately less than 0.001 within the Brewster angles of a material. In addition, the results for refractive index show a reasonable range of retrievals within the Brewster's angle. The imaginary parts of the refractive indexes have larger errors than the real parts, due to the uncertainty of the direct inversion of the Fresnel equation.