2.5 cm wide agate crystal. The picture was not made with a camera, but with a flatbed scanner.
With a birefringent crystal, the optical axis determines the way a light beam is broken through the crystal.
Most crystals are optical anisotropic, i.e. they exhibit birefringence. In such crystals, the refractive index depends on the polarization and the incident direction of the light beam. The optical axis (often called c-axis) is the direction in which each polarization component of a light beam experiences the same refractive index.
Single-axis crystals have one optical axis that is in the direction of the unical refractive index. Two-axis crystals have two optical axes. These lie in the plane stretched by the vectors of the smallest and the largest of the three main indices (the main axes). One of these axes is the mirror image of the other relative to one of the two main axes.
In a unicellular crystal, the beam of light behaves along the optical axis just as in an isotropic crystal. In a two-axis crystal, one beam of light travels along the optical axis, for polarization components in the direction of the middle of the three main axes of the three main axes, a common ray and for all other polarization components an extraordinary radius that occurs for each polarization component in another propagation. Because of the equal refractive indices for all polarization components, all polarization directions are equivalent and no splitting occurs in two rays. Instead, there is a conical refraction of the extraordinary radius. For non-polarized light, this means that there is a radius cone with the optical in the cone sheath.
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