Dichroïsme

Dichroïsme, the French term for dichroism, is the optical property of certain materials to absorb light differently depending on the polarization or the direction of the light. In optics and spectroscopy, a dichroic material may transmit one polarization state more efficiently than another, leading to observable color or brightness changes when viewed through polarized light or as the sample is rotated relative to the light source. Different formulations of the concept distinguish absorption that depends on linear polarization from absorption that depends on circular polarization.

Two principal forms are linear dichroism, where absorption varies with the orientation of linear polarization relative to the material’s anisotropy, and circular dichroism, where absorption differs between left- and right-handed circularly polarized light. In broader usage, dichroism can refer to any polarization-dependent absorption; in mineralogy and gemology, related phenomena are sometimes described as pleochroism, which notes color changes with viewing direction.

Measurement and examples: dichroism is typically studied with spectrophotometry using polarizers or with circular dichroism spectroscopy to probe chiral molecules. Minerals such as certain tourmalines show pleochroism, appearing to change color with different crystal directions; organic dyes, liquid-crystal samples, and biological macromolecules can also exhibit dichroic effects. Circular dichroism is widely used in biochemistry to infer secondary structure in proteins and other biomolecules.

Applications include polarizing filters, sunglasses, and display technologies, as well as analytical techniques in chemistry and biology. Dichroïsme provides a window into molecular orientation, crystal structure, and chiroptical properties, supporting both practical devices and scientific investigations.