Bombardment of a silica‐soda‐lime glass (nD=1.5246) by more than 5×1016A+ions/cm2with an energy of 33.5 kev reduces the reflection coefficient to 0.36 of its normal value for light of wavelength &lgr;=0.6 &mgr;. The change in reflection coefficient is attributed to the formation of an altered glass layer. For a bombardment by 40‐kev A+ions, the layer has a thickness of 0.095 &mgr; and an effective refractive index of 1.343. The depth of the layer is determined by the ion energy and agrees approximately with the theoretical range of the incident ions. The refractive index of the altered layer is determined by the integrated flux of positive ions and the type of ion. The magnitude of the positive ion flux employed here is shown to be orders of magnitude larger than the flux of primary ``knocked‐on'' atoms produced by fast neutron bombardment in a reactor.