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(from Greek pleion, "more," and chros, "colour"), in optics, the selective absorption in crystals of light vibrating in different planes. Pleochroism is the general term for both dichroism, which is found in uniaxial crystals (crystals with a single optic axis), and trichroism, found in biaxial crystals (two optic axes). It can be observed only in coloured, doubly refracting crystals. When ordinary light is incident on a crystal exhibiting double refraction, the light is split into two polarized components, an ordinary ray and an extraordinary ray, vibrating in mutually perpendicular planes. A dichroic substance such as tourmaline transmits only the extraordinary ray, having absorbed the ordinary ray (see ).