Final answer:
Birefringent minerals break light into two plane-polarized rays vibrating perpendicular to each other. One ray behaves normally and is called the ordinary ray, while the other does not obey Snell's law and is called the extraordinary ray.
Step-by-step explanation:
Birefringent minerals break light into two plane-polarized rays vibrating perpendicular to each other. When an unpolarized beam of light passes through a birefringent crystal, it splits into two rays with different values of index of refraction. One ray behaves normally and is called the ordinary ray, while the other does not obey Snell's law and is called the extraordinary ray.
For example, the common mineral calcite is a birefringent material that splits unpolarized beams of light into two. The ordinary ray follows the expected behavior, while the extraordinary ray does not. This phenomenon can be used to produce polarized beams from unpolarized light using birefringent crystals.