Dichroizm

Dichroism is an optical property in which a material absorbs light differently depending on the light's polarization or propagation direction, so colors can change with orientation or polarization. The term comes from Greek di- “two” and chrōma “color.”

There are two common forms. Linear dichroism occurs when absorption depends on the polarization direction relative to the material's optical axes. Circular dichroism occurs when left- and right-handed circularly polarized light are absorbed to different extents. Linear dichroism is common in anisotropic crystals, fibers, and some pigments, while circular dichroism is typical of chiral molecules such as many organic compounds and biomolecules.

Dichroism arises from anisotropy in a material's electronic structure and molecular arrangement, which makes certain electronic transitions more likely for specific polarizations. The strength is often described by a dichroic ratio or differential absorption at a wavelength.

Applications include polarizing filters, sunglasses, and dichroic coatings that impart color effects to glass and jewelry. In science, circular dichroism spectroscopy helps study the secondary structure of proteins and other chiral molecules, while linear dichroism spectroscopy probes oriented samples in materials research.

Related phenomena include pleochroism, the general color change in anisotropic crystals, and birefringence, a difference in refractive index for different polarizations that affects phase velocity rather than absorption.