We are doing an experiment that is called the Raman-Nath experiment, in which we calculate the speed of sound with the interaction between ultrasonic sound waves and light.
The experiment manual can be found here. If you go through the theory of acousto-optics, it says that there is diffraction phenomena occur because the media act as grating due to a sound waves. The same explanation can be found in this paper. But the we are said, that all that is happening is that light is refracted more due to high refractive index part (that is antinode) and less due to low refractive index point and light passes through these and thus we get high and low-intensity region. There is no diffraction because the sound wave length are of the order of $mm$ and wavelength of light is of the order of $nm$. While this is true, It's contradicting the papers and Wikipedia.
Can you please clarify if there is or is not a diffraction phenomenon?
In our experiment, there are standing waves that are formed in liquid. So I would expect the refractive index of the form $$n(z,t)=n_0+2y_{\text{max}}\sin\left(\frac{2\pi x}{\lambda}\right)\cos (\omega t)$$ Due to the change in refractive index, clearly,, there will be a phase difference between adjacent waves. But since refractive index changing rapidly, the phase will also change so we would not able to observe any pattern but since at the intersection part where the light is passing continuously, you would observe a bright fringe.
This is the explanation given, and it doesn't have any diffraction phenomena. Simply saying there are node points where light passes and other points where light doesn't because change in phase.