When light moves from air into glass, it bends, a process called refraction, determined by the material's refractive index.
The refractive index is how much a material slows down light.
Some materials, known as birefringent materials, bend light differently depending on its direction.
This happens because these materials have different crystal structures in different directions.
The way light bends also depends on its polarization, which is the direction of its electric field.
Birefringence can be natural (like in quartz) or artificial.
Birefringent materials are used in LCD screens, medical microscopes, optical switches, waveplates, frequency converters, and high-power lasers
Birefringence can be induced by physical stress, or by exposure to electric or magnetic fields.
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