gel solvent as it sets then we can cause a difference in refractive index between the sites of the acoustic nodes and anti nodes throughout this process the acoustic length of the cavity must not change thus we require a gel solution which has the same acoustic properties as the solid gel the effect on the system of having solid gel of different acoustic properties to its solution would be to remove the standing wave condition and therefore to disturb the solid gel fig6 shows the application of the method to glass a rectangular slab of glass 60 has one end 61 polished to act as a reflector with a transducer 62 coupled to the remote end an oven 63 through which the glass can pass is arranged to heat the glass above its transformation temperature and a longitudinal acoustic wave is introduced through one end of the acoustic cavity to form a phase grating the glass is then cooled while maintaining an acoustic standing wave once cooled below the transformation temperature the acoustic wave is withdrawn and residual stress induced into the glass at the antinode positions of high stress forms a permanent phase grating if we have a glass at.