Birefringence
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Different indices for different polarisations.
Some crystals (calcite, quartz) have different refractive indices for different polarisations, splitting a beam into ordinary and extraordinary rays. Used in wave plates.
Double image through calcite.
Depends on the crystal axis.
In more detail
Birefringent materials have two refractive indices depending on polarisation, splitting an incoming ray into two. Calcite and quartz are common examples.
Real-world uses
- Polarising prisms such as Nicol prisms use birefringence.
- Stress analysis uses birefringence in transparent plastics.
- Calcite crystals show two images of an object, because light splits into two polarised rays with different speeds.
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Frequently asked questions
What is double refraction?
The splitting of an unpolarised ray into two rays with different polarisations.
Why is calcite special?
It shows strong birefringence, so it splits images into two.
What causes birefringence?
Anisotropic crystal structure, which gives the material different refractive indices in different directions.
Related topics in Optics
Malus's law · Brewster's angle · Polarisers and LCDs · Optical activity · Coherence and lasers · Holography