 The Bragg angle is given by the expression,

where formula_18 is the wavelength of the incident light wave (in a vacuum), formula_25 is the acoustic frequency, formula_26 is the velocity of the acoustic wave, formula_27 is the refractive index for the incident optical wave, and formula_28 is the refractive index for the diffracted optical waves.In general, there is no point at which Bragg diffraction takes over from Raman-Nath diffraction.It is simply a fact that as the acoustic frequency increases, the number of observed maxima is gradually reduced due to the angular selectivity of the acousto-optic interaction.