Polarized light, optical principles

In principle, any physical property that varies during the course of the reaction can be used to follow the course of the reaction. In practice one chooses methods that use physical properties that are simple exact functions of the system composition. The most useful relationship is that the property is an additive function of the contributions of the different species and that each of these contributions is a linear function of the concentration of the species involved. This physical situation implies that there will be a linear dependence of the property on the extent of reaction. As examples of physical properties that obey this relationship, one may cite electrical conductivity of dilute solutions, optical density, the total pressure of gaseous systems under nearly ideal conditions, and rotation of polarized light. In sufficiently dilute solutions, other physical properties behave in this manner to a fairly good degree of approximation. More complex relationships than the linear one can be utilized but, in such cases, it is all the more imperative that the experimentalist prepare care-... 

The process by which a stereochemically inactive center is converted to a specific stereoisomeric form. In most cases, the reacting center is prochiral. Such processes can occur with reactions involving an optically active reagent, solvent, or catalyst (eg., an enzyme). The reaction produced by such a process is referred to as an enantioselective reaction. In principle, use of circularly polarized light in photochemical reactions of achiral reactants might also exhibit asymmetric induction. However, reported enantioselectivities in these cases have been very small. 

R.M.A. Azzam, N.M. Bashara Ellipsometry and Polarized Light (North-Holland, Amsterdam, 1977) E. Echt Optics (Addison-Wesley, Reading, 1990) M. Born, M. Wolf Principles of Optics (Pergamon, Oxford, 1970)... 

Asbestos, quartz or other minerals can be analyzed by consideration of mineralogical principles and crystal systems. Polarized light, compensation plates, measurement of angles of extinction and dispersion staining are useful techniques. Optical behavior of a mineral is related to the internal crystal structure of the mineral. Tables of optical constants are useful for mineral identification. The microscope is a powerful tool for analysis that should not be overlooked by the industrial hygiene chemist. 

Compounds that contain an asymmetric substitutions on a carbon atoms exist in enantiomorphic forms. These differ only in the direction of rotation of polarized light. When the absolute configuration has been identified in the original report, these compounds are identified by (R)- and (S)- prefixes. Sometimes prefixes such as d-, /-, (+)-, and (-)-, which are given in the original report have been used. Very often no identification of the optical isomer is given even when the compound contains an asymmetric carbon atom Then it is assumed that the samples are racemic mixtures. However, none of these distinctions affect the densities of liquids, therefore all the isomers of this type are combined to determined the recommended values. If a compound contains more than one asymmetric carbon atom diasteieomers can exist. In principle, these might have different densities, but no examples have been found. 

The two compounds differ physically only in the orientation of their crystals and the direction in which their solutions rotate polarized light. These may seem like trivial differences, but when such optical isomers interact with other asymmetric molecules (especially enzymes) they may acquire very different chemical reactivities. This principle of optical selectivity is central to the biochemistry of carbohydrates (and other classes of naturally occurring compounds as well). The manner of drawing the two isomers as shown in the top part of Figure III-I is called the Fischer projection. In this projection, the most oxidized (aldehyde) end of the molecule is drawn up and is considered to be behind the plane of the paper. The -CH2OH end is drawn down and is also behind the plane. Because the groups are... 

Optical biosensors can be defined as sensor devices which make use of optical principles for the transduction of a biochemical interaction into a suitable output signal. The biomolecular interaction on the sensor surface modulates the light characteristics of the transducer (i.e., intensity, phase, polarization, etc.), and the biosensing event can be detected by the change in diverse optical properties such as absorption, fiuorescence, luminescence or refractive index, among others. 

Let us adjust the lenses so that a maximum amount of light is allowed to pass. (In practice, it is easier to detect a minimum than a maximum the principle remains the same.) Now- let us pkicc the sample to be tested in the lube. If the substance does not affect the plane of polarization, light transmission is still at a maximum and the substance is said to be optically inactive. If, on the other hand, the substance rotates the plane of polarization, then the lens nearer our eye must be rotated to conform w ilh this new plane if light transmission is again to be a maximum, and the substance is said to be optically active. If the rotation of the plane, and hence our rotation of the lens, is to the right (clockwise), the substance is dextrorotatory (Latin dexter, right) if the rotation is to the left (counterclockwise), the substance is levorotatory (Latin laerus, left). 

Active Q-switching occurs when laser light access to one of the mirrors in the cavity is controlled electro-optical cell, which works on the principle of affecting the passage of polarized light (see... 

Applying these principles, scientists have developed an instrument called a polarimeter that is used to measure the optical activity of molecules. Specifically, the polarimeter measures the ability of a compound to change the angle of the plane of plane-polarized light (refer to Figure 17.5c). 

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