uniaxial crystals from isotropic media is derived by using Huyghens principle. And the matched

sign convention is also given. The refrac- tion relation is appropriate for the situations where

the optical axis is in the incidence plane. Three special cases are discussed with the refraction

relation. The results are as follows: when the incident light ray rotates around the inci- dence

point in the incident plane，the extraordinary light ray must rotate in the same direction; under

the particular conditions，the extraordinary light ray can appear at the side of the

interfacial normal where the incident light ray exists; the refraction angle of the

extraordinary light ray has a maximum value when the incident angle is 90°.