43. Phenomenon of double refraction. Dichroism.

Double Refraction or Birefringence is an optical property in which a ray of light entering a medium is decomposed into two rays, each traveling in a different direction. One ray is bent or refracted called the extraordinary ray and the other passes through the medium unchanged called the ordinary ray. This effect can occur only if the structure of the material is anisotropic. Anisotropy is the property of a material to be directionally dependent due to which a material does not behave the same way in all directions. When anisotropic crystals refract light, the resulting rays are polarized along the axis of anisotropy and travel at different velocities giving rise to the phenomenon of double refraction. One of the rays travels with the same velocity in every direction (ordinary ray). The other ray travels with a velocity that is dependent upon the propagation direction (extraordinary ray). The distance of separation between the ordinary and extraordinary ray increases with increasing crystal thickness. The two independent refractive indices of anisotropic crystals are quantified in terms of their birefringence(double refraction), a measure of the difference in refractive index. Thus, the birefringence (B, often termed d, or D) of a crystal is defined as: B = |n(high) – n(low)|, where n(high) is the largest refractive index and n(low) is the smallest. This expression holds true for any part or fragment of an anisotropic crystal with the exception of light waves propagated along the optical axis of the crystal.(B, often termed d, or D) of a crystal is defined as: B = |n(high) – n(low)|, where n(high) is the largest refractive index and n(low) is the smallest. This expression holds true for any part or fragment of an anisotropic crystal with the exception of light waves propagated along the optical axis of the crystal.

44. Research of microstructures in polarizing light.

At the heart of instruments used for research in polarized light on the system of the polarizer and analyzer, LAYOUT conjugated along the direction of light beams, which are placed between the object under study. The analyzer is designed like a polarizer, but is adapted to rotate about the longitudinal axis.

If the plane of the polarizer P and the analyzer A coincide, the light passes completely through Analyzer and forms on-screen e bright spot.

By turning the analyzer brightness spots on the screen decreases. Descending as the light intensity I passing through analyzer comes ratio (Malus law) light intensity, where the intensity of the light transmitted through the polarizer and the analyzer, respectively, and α - angle of rotation of the plane anali mash. When mutually perpendicular arrangement of the planes of the polarizer and analyzer light is completely extinguished analyzer. Thus, in one complete revolution (360 °) of the analyzer screen is fully illuminated and double twice completely obscured. Polarized light used in the study of optical anisotropic elements again individual structures, in particular body tissues. In many cases, it is possible to establish races position and structure of the structural elements, which are not detected under the microscope in the natural Venn light. for example, muscle, connective tissue (collagen) nerve fibers. Study of muscle fibers in polarized light reveals that darker areas are anisotropic, while lighter – isotropic.