Birefringence
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Birefringence, also called double refraction, is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically aniso
A calcite crystal laid upon a graph paper with blue lines showing the double refraction
View from under the Sky Pool, London with coloured fringes due to stress birefringence of partially polarised skylight through a circular polariser
Sandwiched in between crossed polarizers, clear polystyrene cutlery exhibits wavelength-dependent birefringence
Color pattern of a plastic box with "frozen in" mechanical stress placed between two crossed polarizers
Iceland spar
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Iceland spar, formerly called Iceland crystal and also called optical calcite, is a transparent variety of calcite, a crystallized calcium carbonate, originally brought from Iceland and used in demons
Birefringence
…into two rays taking slightly different paths. This effect was first described by Danish scientist Rasmus Bartholin in 1669, who observed it in Iceland spar (calcite) crystals, which have one of the strongest birefringences. In the 19th century, Augustin-Jean Fresnel described the phenomenon in terms of…
Calcite rhombohedral crystal structure
Calcite crystal birefringence
Iceland spar, possibly the Icelandic medieval sun stone used to locate the sun's direction in overcast or foggy skies