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In certain anisotropic materials, the property of having different absorption coefficients for light polarized in different directions. There are few natural materials which exhibit strong dichroism. One of the first to be discovered was tourmaline. Light transmitted by thin plates of dark forms of tourmaline is almost completely polarized. See Polarized light
If the absorption in a dichroic material is different for different linear states of polarization, the material is termed linear dichroic. If it is different for right and left circularly polarized light, it is termed circular dichroic. Similarly, there can be elliptically dichroic crystals.
The study of dichroism allows conclusions as to the submicroscopic fine structure of cells. In visible light only a few cellular components, such as chloroplasts, show absorption. An absorption can, however, be produced by staining. The dichroic staining of plant fibers is especially simple. The elongate stain particles of benzidine dyes, for example, congo red, are deposited in an oriented manner in the spaces between the microfibrils and produce an intrinsic dichroism of the fiber: colored for a vibration plane parallel, colorless for a plane perpendicular to the stain particles and fibrils. Therefore, the direction of strongest absorption indicates the course (parallel or helical) of the microfibrils in the fiber.
Ultraviolet dichroism gives direct information as to the orientation of the absorbing molecules or molecular groups in cell structures. The method has been especially helpful for studies of orientation of deoxyribonucleic acid in nuclei and chromosomes. Lignifed plant cell walls show ultraviolet dichroism. It is pure form dichroism, a fact which eliminates the possibility that lignin is in an anisotropic state in the wall.
By irradiation with ultraviolet light, various compounds of the cell are caused to fluoresce. The fluorescent light is polarized if the object is anisotropic. This phenomenon, called difluorescence, is observable in lignifed cell walls, and leads to the same conclusions as to lignin deposition as emerge from dichroism studies.
a difference in the color of uniaxial crystals that have double refraction (birefringence). The difference in color is observed in transmitted light for two directions of viewing—along and perpendicular to the optical axis. For example, an apatite crystal illuminated by a white light appears to be light yellow if viewed along the optical axis and green if viewed perpendicular to this axis. For such conditions of observation the coloring of the crystal is called axial or basic, respectively. When viewed in some other direction, the crystal exhibits some intermediate color—that is, dichroism is a special case of pleochroism.
Dichroism is due to the fact that the absorption spectra of a crystal are different for light rays of different direction and polarization.