English to Portuguese Tech/Engineering Mining & Minerals / Gems

birefringence: The splitting of incident light into two rays vibrating at right angles to each other, and causing two images to appear, eg in calcite, caused by double refraction in birefringent materials whose refractive index varies with direction. Arises from anisotropy in the material, eg crystalline anisotropy, molecular orientation, **frozen-in or imposed strains. The effect is used in mineralogy, in stress analysis and to determine degree of orientation in plastics and textile fibres.