Polarized light optical microscope

Isotropic materials have properties which are independent of the direction of examination, X-, y- or z-direction. As such, they appear with a purple color in the polarized light optical microscope and this color does not change on rotation of the stage of the microscope. Thus, reflectance and bireflectance are not relevant properties for isotropic carbons. The porous, isotropic carbons are easily identifled and distinguished from anisotropic carbons by this technique. 

Sap of lacquer trees, which appears dark under a polarized light optical microscope, showed a "Maltese cross" pattern with a number of circles of about 10 ym when it was dried into a film as shown in Fig. 9. This indicated the formation of considerable molecular orientation of the plant gum in the film, corresponding in pattern to the water droplets in the oil(urushiol) phase of sap. 

Fig. 12 Polarized light optical microscope observation of VAzPy C12 at its LC phase upon UV irradiation. The right picture was obtained after irradiation with visible light ftw 80 s. Reproduced with permission from Chen et al. [61]. Copyright 2014, Royal Society of Chemistry...

Occasionally an optically inactive sample of tartaric acid was obtained Pasteur noticed that the sodium ammonium salt of optically inactive tartaric acid was a mixture of two mirror image crystal forms With microscope and tweezers Pasteur carefully sep arated the two He found that one kind of crystal (m aqueous solution) was dextrorota tory whereas the mirror image crystals rotated the plane of polarized light an equal amount but were levorotatory... 

Optical properties also provide useful stmcture information about the fiber. The orientation of the molecular chains of a fiber can be estimated from differences in the refractive indexes measured with the optical microscope, using light polarized in the parallel and perpendicular directions relative to the fiber axis (46,47). The difference of the principal refractive indexes is called the birefringence, which is illustrated with typical fiber examples as foUows. Birefringence is used to monitor the orientation of nylon filament in melt spinning (48). 

There is the microtoming optical analysis test. In this procedure thin slices (under 30 tixri) of the plastics are cut from the product at any level and microscopically examined under polarized light transmitted through the sample. Rapid quality and failure analysis examination occurs by this technique. This technique has been used for many years in biological studies and by metallurgists to determine flaws, physical and mechanical properties. Examination can be related to stress patterns, mechanical properties, etc. 

An optical microscope photograph taken at 200 X magnification using polarizing filters is shown in Fig. 21. The spherulites show a characteristic Maltese cross pattern produced by the interaction of the polarized light with the... 

The first example of the deliberate separation of optically active molecules is appropriate as an example of physical separation in the clearest sense of the term. The molecules are referred to as optically active because polarized light interacts differently with right- and left-handed molecules. In the case of simple diastereomers the RR and SS forms are enantiomers while the RS and SR forms are not. The separation of the latter and former was first done under a microscope using crossed polarizers and the crystals which were seen were separated from those that caused little or no rotation of plane-polarized light by hand using tweezers. A truly physical separation of chemical species using a physical property of chemical origin ... 

Fig. 28 Image of a crystallization nucleus triggered by a pigment particle in polyethylene in polarized light, taken with an optical microscope.

Table VI shows the results of polarized light microscopic observations. Sometimes isotropic regions and the middle phase exist simultaneously. The region of the middle phase is marked by heavy lines. The range of the especially viscous middle phase narrows with transition from two to three oxyethylene groups in the surfactant molecule. Up to 27 %, the system appears optically isotropic. In this concentration range the viscosity can be increased strongly by addition of NaCl, as shown in table VII.

However, if an LC substance is heated, it will show more than one melting point. Thus, liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid and a solid crystal. For instance, an LC may flow like a liquid but have the molecules in the liquid arranged and/or oriented in a crystal-like way. There are many different types of LC phases that can be distinguished based on their different optical properties (such as birefringence). When viewed under a microscope using a polarized light source, different liquid crystal phases will appear to have a distinct texture. Each patch in the texture corresponds to a domain where the LC molecules are oriented in a different direction. Within a domain, however, the molecules are well ordered. Liquid crystal materials may not always be in an LC phase (just as water is not always in the liquid phase it may also be found in the solid or gas phase). 

Common liquids are optically isotropic, and the solids that physicists seem to like most are cubic and therefore isotropic. As a consequence, treatments of optical properties, particularly from a microscopic point of view, usually favor isotropic matter. Among the host of naturally occurring sohds, however, most are not isotropic. This somewhat complicates both theory and experiment for example, measurements of optical constants must be made with oriented crystals and polarized light. But because of the prevalence of optically anisotropic solids, we are compelled to extend the classical models to embrace this added complexity. 

We shall first examine the microscopic techniques which allowed us to study these transformations and to show the striking analogy between the images obtained by optical microscopy in polarized light and by electron microscopy with ultrathin sections, despite the difference of the absorption mechanisms of light and electrons. Once this analogy was established, we sought to use electron microscopy and electron microdiffraction to learn more about the texture and structure of the anisotropic areas. 

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