From Online Dictionary of Crystallography

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Pléochroïsme (Fr). Pleochroismus (Ge). Pleocroísmo (Sp). Pleocroismo (It). 多色性 (Ja).

Pleochroism is the phenomenon in which a substance appears to be different colours when observed at different angles under polarized light. It finds its origin in the fact that the optical properties of anisotropic crystals vary with the direction of light. The polarization of light determines the direction of the electric field; anisotropic crystals respond differently if this angle is changed. When the absorption of light varies with the angle relative to the optical axis, then pleochroism results. The pleochroic colours are at their maximum when light is polarized parallel to a crystallographic axis.

  • Uniaxial crystal (tetragonal, trigonal and hexagonal) can only show two colours; pleochroism is then called dichroism.
  • Biaxial crystals (orthorhombic, monoclinic and triclinic) can show three colours; pleochroism is called then trichroism.