Kindly go through the link for measurement using interferometer

http://vlab.amrita.edu/?sub=1&brch=189&sim=1519&cnt=1,

http://physlab.org/wp-content/uploads/2016/04/Refractive_index.pdf

Dear Ravi

Thanks to your response.

and what is the advantage and disadvantage of this method related to diffraction and polarization method of measuring refractive index of plates?

Article Fabry-Perot laser interferometry to measure refractive index...

If you have any a prori information about the plate, the experimental setup can be extremely simple. This simplicity is related with the (a priori known) optical properties of the sample and the question of which method is better (diffraction, polarization, etc) can be answered.

For example, if the plate is a piece of isotropic material, an extremely simple interferometer can be useful: Article Single-element interferometer

If your sample is a piece of anisotropic (birefringent) material, maybe you can use this simple polarization interferometer: Article Birefringent phase demodulator: Application to wave plate ch...

Regards,

Lucas.

thanks a lot dear Jose

dear Lucus, thanks a lot for your answer