The splitting between longitudinal and transverse optical modes was observed in the infrared reflection spectra of silica glasses at different incident angles. The density of states and the frequency of the longitudinal mode were found to increase with increasing incident angle. A sharp minimum in the reflectance was monitored at large incident angles. A transmission window was observed in the absorption spectra of silica glass plates. The sharp minimum in the reflectance and appearance of the transmission window are believed to be the results produced by the Doppler effect due to the interaction of longitudinal and transverse optic modes in silica glass. The phase velocity of the longitudinal mode was evaluated in terms of the relativistic Doppler shift formula. (C) 2000 American Institute of Physics. [S0021-9606(00)71713-2].