NATURAL HISTORY MUSEUM, LONDON / SCIENCE PHOTO LIBRARY NATURAL HISTORY MUSEUM, LONDON / SCIENCE PHOTO LIBRARY
Double refraction (birefringrnce) in a calcite crystal. This phenomenon, which is similar to polarisation, splits the light ray into two components, the fast or ordinary ray and the slow or extraordinary ray. Because the two types of waves travel at different speeds, they get out of phase, producing the double images behind the crystal. Calcite consists almost entirely of calcium carbonate. It is the main constituent of limestone and marble. This is a specimen if Iceland spar from Rodefjord, Iceland.
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