 This then gives rise to the variation of the refractive index.For a plane acoustic wave propagating along the z axis, the change in the refractive index can be expressed as,

where formula_11 is the undisturbed refractive index, formula_12 is the angular frequency, formula_13 is the wavenumber of the acoustic wave, and formula_14 is the amplitude of variation in the refractive index generated by the acoustic wave, and is given as,

The generated refractive index, (2), gives a diffraction grating moving with the velocity given by the speed of the sound wave in the medium.Light which then passes through the transparent material, is diffracted due to this generated refraction index, forming a prominent diffraction pattern.