Jamin interferometer

The Fresnel reflection from the first surface of the mirror acts as a beam splitter.

The incident light is split into two rays, parallel to each other and displaced by an amount depending on the thickness of the mirror.

If a phase-shifting element is added to one arm of the interferometer, then the displacement it causes can be determined by simply counting the interference fringes, i.e., the minima.

The Jamin interferometer allows very exact measurements of the refractive index and dispersion of gases; a transparent pressure chamber can be positioned in the instrument.

The phase shift due to changes in pressure is quite easy to measure.