Having the property of splitting light rays in two different directions; showing double refraction.
From Latin 'bi-' (two) and 'refract' (from Latin 'refringere' meaning to break). This optical term became established in the 18th century as scientists studied crystal properties.
Iceland spar (calcite crystal) is famous for birefracting light—Vikings may have used it as a navigation tool because it shows two images simultaneously, helping them navigate when the sun was hidden by fog or clouds.
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