Optical properties in general

In general the interaction of electromagnetic radiation (light) with matter is controlled by three properties  

These properties are directly related to the refractive index and the extinction index of the medium. 

It is sometimes useful to distinguish between conducting or dissipative media (crel 0) and non-conducting media (cej = 0) For non-conducting media the velocity of an electromagnetic wave is proportional to (p.e)1/2, just as the velocity of a sound wave is proportional to the square root of the compressibility. 

Among the optical properties refraction, absorption, reflection and scattering of light are the most important. While the first three properties are determined by the average optical properties of the medium, scattering is determined by local fluctuations in optical properties within the medium. 

Light is changed in phase in traversing a medium some light may be lost from the transmitted beam by extinction. Both the phase change and the extinction may be described by a complex refractive index  

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More information on color and optical properties in general is in ... 

To sum up, while there is too little information available to draw any firm conclusions, it appears that films deposited from most thiourea baths are weakly absorbing in the near-IR region and that films deposited from thiosulphate solutions (which are mildly acidic) may possess different optical properties in general than those deposited from (alkaline) thiourea baths. In this respect, and if this difference is real, it would be interesting to deposit PbS from thioacetamide baths, which can be both acidic and alkaline. 

Apart from high strength materials formed from nematic polymer fibres, most applications of nematic liquid crystals depend on their anisotropic optical properties. As a consequence the refractive indices of nematics are of prime importance in the development of materials for applications. The refractive indices are determined by the molecular polarisability coupled to the mientational order of the mesogens in the liquid crystal phase, so refractive indices can provide a direct probe of the order parameter. Furthermore the optical properties of liquid crystal films are frequently used to determine phase behaviour, identify phase types through characteristic optical textures or explore the properties of defects, and such experiments rely on the anisotropy in the refractive index of the material. The first tool of a liquid crystal scientist is the polarising microscope, which emphasises the importance of optical properties in general and refractive indices in particular to the stufy of liquid crystals. 

Refractive index is a basic optical properly of fibers that is directly related to other optical properties. In general, the refractive index of fibers varies with temperature and wavelength. The standard conditions for refractive index measurement involve the use of specific wavelength (589 mn) at a specific temperature (20°C). 

Within the limitations on the physical properties which generally restrict plastics to low precision optics, plastics materials have found wide applications in optical products that range from lights to binders for electroluminescent phosphors to fiber optics and lasers. They represent an easily worked material with a wide range of desirable optical properties in simple to complex shapes. In this review the discussion has been limited to the differences between plastics and optical glass materials and to some of the unique design possibilities that are especially important for plastics. Using the optical arts and the... 

Optical sensing in general is based on changes of the optical properties of materials within the optical circuitry due to changes in the environment. 

Many inert pigments (often known as fillers) are incorporated into paper in addition to the cellulosic fibres. They may be added to improve certain optical properties—in particular opacity and brightness—or simply as a cheap replacement for costly fibre. The two most common pigments are kaolin (china clay) and chalk (limestone), but talc and speciality pigments such as titanium dioxide are also used. The particle size for general purpose fillers is normally expressed as an equivalent spherical diameter (esd) and this is determined from sedimentation data. Values for the common paper-... 

The empirical approach [7] was by far the most fruitful first attempt. The idea was to fit a few Fourier coefficients or form factors of the potential. This approach assumed that the pseudopotential could be represented accurately with around three Fourier form factors for each element and that the potential contained both the electron-core and electron-electron interactions. The form factors were generally fit to optical properties. This approach, called the Empirical Pseudopotential Method (EPM), gave [7] extremely accurate energy band structures and wave functions, and applications were made to a large number of solids, especially semiconductors. [8] In fact, it is probably fair to say that the electronic band structure problem and optical properties in the visible and UV for the standard semiconductors was solved in the 1960s and 1970s by the EPM. Before the EPM, even the electronic structure of Si, which was and is the prototype semiconductor, was only partially known. 

For optically uniaxial crystals we know that the refractive index values for extraordinary waves are variable, with that for ordinary waves fixed. We can link this observation with that concerning the vibration directions for the two waves travelling along a general wave normal direction the ordinary vibration direction is always perpendicular to the optic axis, while the extraordinary vibration is always in the plane containing the optic axis and wave normal direction. This suggests that we may connect the variation of the refractive index in the crystal with the vibration direction of the light. This concept allows a convenient representation of anisotropic optical properties in the form of a spatial plot of the variation of refractive index as a function of vibration direction. Such a surface is known as the optical indicatrix. 

An overview of these topics is given in [26] for optical films in general. The following subsections refer to ZnO deposition in more detail, taking also the specific property of Zn, its extraordinarily high vapor pressure, into account. 

The anisotropy of the optical properties in the wavelength range of visible light is generally described by the birefringence An, defined by the first part of Equation 8.12. A birefringent material has different refractive indices in two different principal axis directions. For a specimen that is under uniaxial tension, nn and n denote the refractive indices parallel and perpendicular to the orientation direction, respectively. The second part of Equation 8.12 defines the stress-optic coefficient C0 describing the dependence of An on the applied stress. 

Colour determination can be carried out with several reference methods, generally based on the sample optical properties in the visible region (Table 2). The examination of the different procedures leads to the conclusion that, except for the USEPA 1 method, which uses several sets of three wavelengths, the others are limited to the choice of the wavelengths to be considered and give less useful results, which are apparently not very close to the significance of the parameter. Almost all methods can be automatically performed by a PC-controlled spectrophotometer, provided the bandwidth of the instrument is adapted for the measurement. 

When a crystal is subjected to a stress field, an electric field, or a magnetic field, the resulting optical effects are in general dependent on the orientation of these fields with respect to the crystal axes, it is useful, therefore, to express the optical properties in terms of the refractive index ellipsoid (or indicatrix) ... 

Several reports in the literature have suggested metal-metal interactions in the solid state for metal isocyanide complexes (53, 162, 220, 221, 282). Materials of the type Ma2(CNR)2(M = Pt, R = alkyl, aryl) have been prepared that exhibit different optical properties in the solid state than in solution. This phenomenon is generally indicative of metal-metal interactions. Since partial oxidation is not apparent from the stoichiometry, low conductivity is anticipated. The studies were limited to some vcifjt absorption (221) and reflectivity data (162) in the solid state. Structures SO, 31, and 32 (or mixtures thereof) fit the observed stoichiometry. 

For Eg O (nonmetal), the optical properties can generally be successfully modeled using the particle in a sphere model and its extensions." " Consideration of an electron and hole with negligible spatial correlation in a potential well, with neglect of polarization effects, leads to the following expression for the shift of the optical bandgap (l hlAe excited state) with the radius of the sphere ... 

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