Iceland spar

Iceland spar is a transparent, colorless variety of calcite with strong birefringence: it divides an entering light beam into two rays with different polarizations.

Birefringence: The splitting of light into rays that travel at different speeds and may follow different paths through a material. Iceland spar is a familiar mineral example of this optical property.
Calcite: A crystalline form of calcium carbonate, CaCO₃, found in many geological settings. Iceland spar is a particularly clear variety of this mineral.
Nicol prism: An optical polarizer made from calcite crystals cut and joined to transmit one polarized ray while rejecting the other. It exploits calcite's double refraction to produce polarized light.
Helgustaðanáma: A historic calcite mine in eastern Iceland, renowned for large, clear crystals of Iceland spar. The mine is the classic source behind the mineral's geographic name.
Isotropic material: A material whose physical properties are the same in every direction. Unlike isotropic glass, Iceland spar has direction-dependent refractive behavior.
Double refraction: The production of two refracted rays when light passes through certain anisotropic materials. Its two visible images arise because incoming light follows two distinct refracted paths.
Crystal optics: The study of how light propagates and interacts with crystalline materials. It supplies the framework for describing Iceland spar's direction-dependent refraction.
Polarimetry: The measurement of the rotation or other properties of polarized light as it passes through a sample. Calcite prisms made controlled polarization available for optical measurements.
Rasmus Bartholin: A Danish physician and mathematician who published an early account of double refraction in calcite in 1669. His observations of Iceland spar brought its striking optical effect to scientific attention.
Quartz: A crystalline form of silicon dioxide that occurs in several varieties and has diverse optical properties. Quartz can also be birefringent, but its optical behavior differs from calcite's strong splitting.

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