![]() ![]() Although similar to the common brightfield microscope, the polarized light microscope contains additional components that are unique to instruments of this class. ![]() The microscope illustrated in Figure 1 is equipped with all of the standard accessories for examination of birefringent specimens under polarized light. Also investigated in polarized light are stresses in transparent singly refracting media (for example, glass) and the identification and characterization of a wide spectrum of anisotropic substances through their refractive index and birefringence. This technique is useful for orientation studies of doubly refracting media that are aligned in a crystalline lattice or oriented through long-chain molecular interactions in natural and synthetic polymers and related materials. Anisotropic substances, such as uniaxial or biaxial crystals, oriented polymers, or liquid crystals, generate interference effects in the polarized light microscope, which result in differences of color and intensity in the image as seen through the eyepieces and captured on film, or as a digital image. Polarized light microscopy is utilized to distinguish between singly refracting (optically isotropic) and doubly refracting (optically anisotropic) media. The colors observed under illumination with white light in the microscope eyepiece can be utilized to quantitatively draw conclusions about path differences and specimen thickness values when the refractive indices of the specimen are known. Interference between the recombining white light rays in the analyzer vibration plane often produces a spectrum of color, which is due to residual complementary colors arising from destructive interference of white light. Because interference only occurs when polarized light rays have an identical vibration direction, the maximum birefringence is observed when the angle between the specimen principal plane and the illumination permitted vibrational direction overlap. If the specimen orientation is altered by 45 degrees, incident light rays will be resolved by the specimen into ordinary and extraordinary components, which are then united in the analyzer to yield interference patterns. This is due to the fact that when polarized light impacts the birefringent specimen with a vibration direction parallel to the optical axis, the illumination vibrations will coincide with the principal axis of the specimen and it will appear isotropic (dark or extinct). During rotation over a range of 360 degrees, specimen visibility will oscillate between bright and dark four times, in 90-degree increments. When the specimen long axis is oriented at a 45-degree angle to the polarizer axis, the maximum degree of brightness will be achieved, and the greatest degree of extinction will be observed when the two axes coincide. When an anisotropic specimen is brought into focus and rotated through 360 degrees on a circular polarized light microscope stage, it will sequentially appear bright and dark (extinct), depending upon the rotation position. After exiting the specimen, the light components become out of phase with each other, but are recombined with constructive and destructive interference when they pass through the analyzer. The velocities of these components are different and vary with the propagation direction through the specimen. Image contrast arises from the interaction of plane-polarized light with a birefringent (or doubly-refracting) specimen to produce two individual wave components that are each polarized in mutually perpendicular planes. In order to accomplish this task, the microscope must be equipped with both a polarizer, positioned in the light path somewhere before the specimen, and an analyzer (a second polarizer), placed in the optical pathway between the objective rear aperture and the observation tubes or camera port. The polarized light microscope is designed to observe and photograph specimens that are visible primarily due to their optically anisotropic character. ![]()
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