The refractive index of silicon has been measured at elevated temperatures at several different laser wavelengths of the HeNe and the Ar+cw lasers. This data, along with the reinterpretation of polarization modulation ellipsometry data, shows that the refractive index is linear with temperature (from 25 to ∼750 °C) and that the temperature coefficient of the refractive index increases as the wavelength of light decreases.