Abstract: A calculation method for phase difference in interference with an anisotropic medium parallel plate is proposed. It has been proved that fringes of extraordinary waves are ellipses. The ratio of two semi-axes of the ellipse is equal to the ratio of two principal refractive indices. The experiment with a lithium niobate crystal has been completed and which is in agreement with the theoretical result.

Key words: parallel plate interference, anisotropic medium, refractive index measurement