Abstract: The refraction relation of the extraordinary light ray with the incident light on

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°.

Key words: uniaxial crystals, extraordinary light, refraction relation