Refractive index optical anisotropy

Optical microscopy is another method that has been used to determine the distribution of minerals in coal. This method is based on the detailed microscopic examination of polished or thin sections of coal in transmitted and/or reflected light. In principle, observing several of its optical properties, such as morphology, reflectance, refractive index, and anisotropy, makes identification of a mineral type possible. 

Molecular labeling is possible. By appropriate manipulations, an optical experiment can sense the behavior of one selected component in a multicomponent fluid. For example, molecules can be labeled by refractive index, polarizability anisotropy, isotopic substitution, or chromophore attachment. This kind of specificity is impossible to achieve in a mechanical experiment. 

Optical Properties. When light falls on an object, it is either partially absorbed, reflected, or transmitted. The behavior of the object as it relates to each of these three possibiUties determines visual appearance. Optical properties of fibers give useful information about the fiber stmcture refractive indexes correlate well with fiber crystalline and molecular orientation and birefringence gives a measure of the degree of anisotropy of the fiber. 

Chromophores with a rather high optical anisotropy are the merocyanines (77), especially in the cyanine limit with equal contributions of the apolar and zwitterionic resonance structures [319]. Thus, they also have been proposed as promising candidates for photorefractive systems based on molecular glasses. For 77, doped with a photosensitizer, a refractive index modulation of 0.01 at an electrical field of 22 V/pm was reported. 

Note 1 The nematic liquid crystal must have a negative dielectric anisotropy (Af < 0), and a positive anisotropy (Aa > 0). The optical texture corresponding to the flow pattern consists of a set of regularly spaced, black and white stripes perpendicular to the initial direction of the director. These stripes are caused by the periodicity of the change in the refractive index for the extraordinary ray due to variations in the director orientation. 

Like cellulose, nitrocellulose demonstrates optical anisotropy (double refraction) which is regarded as one of the facts confirming the microcrystalline structure of the substance. A relationship between this property of nitrocellulose and the nitrogen content has been discovered by Ambronn [127] who studied the double refraction of cellulose. Differences in the refractive index for rays of different wavelengths in two directions at right angle are of the order —0.50 x 10-3 to +2.8 x 10"3 (Table 60). 

Methods used to obtain conformational information and establish secondary, tertiary, and quaternary structures involve electron microscopy, x-ray diffraction, refractive index, nuclear magnetic resonance, infrared radiation, optical rotation, and anisotropy, as well as a variety of rheological procedures and molecular weight measurements. Extrapolation of solid state conformations to likely solution conformations has also helped. The general principles of macromolecules in solution has been reviewed by Morawetz (17), and investigative methods are discussed by Bovey (18). Several workers have recently reexamined the conformations of the backbone chain of xylans (19, 20, 21). Evidence seems to favor a left-handed chain chirality with the chains entwined perhaps in a two fold screw axis. Solution conformations are more disordered than those in crystallites (22). However, even with the disorder-... 

It is important to emphasize that the above discussion does not take into account the optical anisotropy of the oriented films (5), and assuming the same refractive index for all the sample. However, if the films have the same molecular density, such as, for the first approximation, complete monolayers (5-7), these constraints are not critical. On the other hand, these considerations may be important in partial monolayers, since significant deviations may arise when going down in surface concentration. The refractive index of the film, n2, is a monotonic function of the molecular density, varying from 1.00, in the limit of zero surface coverage, to about 1.50, in a complete closely-packed monolayer. 

In an important series of papers [6,7], Jones established an approach for the treatment of materials where the refractive index tensor, n (z), varies along the propagation direction of the transmitted light. This procedure also lays the foundation for the analysis of complex systems possessing any combination of optical anisotropies. 

The term birefringence indicates an anisotropy of some kind in the real part of refractive index n exhibited by a beam of electromagnetic radiation after it traverses a medium. The best known example of birefringence, natural optical activity (NOA), was discussed in another contribution to this book by Pecul and Ruud. 

A linear birefringence involves the occurrence of an anisotropy between the components of the refractive index associated with linearly polarized monochromatic light whose polarization vector is directed along two perpendicular optical axes. In the examples discussed here the principal optical axis lies parallel to an external applied field, and... 

Formula (4.370) leads to a simple expression for the macroscopic optical anisotropy. If the Oz axis of the coordinate framework coincides with the applied held direction h, then the uniaxial symmetry condition yields (n2) = (n2) and vx = vy. Hence for the refractive index anisotropy (birefringence) we have... 

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