The temperature dependence of refractive indices (dn/dT) for crystalline and amorphous silicon is analyzed critically by use of a recently introduced physically meaningful model to find refractive indices at any operating temperature and wavelength. The opposite sign of the measured thermo‐optic coefficients of the amorphous silicon at 0.6328 and 0.752 &mgr;m is investigated and is explained as due to the inverted position of the isentropic band gap rather than the crystalline silicon. ©1995 American Institute of Physics.