The infrared reflection spectra of silica glass were monitored at different angles of incidence and different polarization directions of the incident light. The frequency of the transverse optic mode in the vicinity of 1129 cm(-1) remains a constant under different conditions, whereas the frequency of the longitudinal optic mode near 1260 cm(-1) and the ratio R-LO / R-TO are functions of the angle of incidence and polarization direction of the plane polarized incident light, where R denotes the reflectance. These observations are due to the gradient in the vibration amplitude in the direction perpendicular to the glass surface. (C) 2003 American Institute of Physics.