Double refraction
Each ray has two waves
When a ray of light arrives at an anisotropic crystal, it splits into two rays of polarised light which vibrate in perpendicular planes.One ray obeys the laws of refraction (ordinary ray) and the other does not (extraordinary ray).

Additionally, both present different refractive indices (their directions of vibration are different). When they leave the crystal, both follow parallel paths even though their planes of vibration continue to be perpendicular.
The ordinary and extraordinary component follow different paths inside the crystal, but on leaving the crystal, they follow parallel paths. To simplify matters, we can assume that both components follow a single path although they are vibrating in perpendicular planes.