Fabry-Perot laser interferometry to measure refractive index or thickness of transparent materials

By introducing a transparent plane parallel plate into an open Fabry-Perot cavity and counting the change in the interference order while the plate is rotated, it is possible to measure the thickness or refractive index of the material with a high degree of accuracy. The method of multipass interferometry can be applied by using a He-Ne laser of 632.8 nm wavelength.