Types of Prisms

Glass prisms do not require the special attention given quartz since optical rotation and birefringence do not occur in glasses. However, glass prisms for spectrometers commonly are made similar to those of quartz. The 60° prism is common, as is a Littrow (half-prism) type. Prisms usually are used near the position of minimum deviation. For most uses the 60° prism appears to be close to the optimum design. 

If the two mirrors are equidistant from the beam splitter, the amplitudes of the two beams combine constructively. If one mirror is moved a distance of A/4, the emerging beam will be the result of two beams 180° out of phase and they will combine destructively. If the entering beam from the source is monochromatic, the detector will observe a cosine signal whose amplitude is a function of the mirror position. If the entering beam is polychromatic, the signal observed at the detector is a summation of all interferences as each 

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Observed first-order reflections from planes with one or two indices even, with the sum of all three indices even, and with the sum of any two indices even (Table II) require6 that the lattice underlying the structure be the simple orthorhombic lattice To- The types of prism planes giving first-order reflections (Table III) are such as to eliminate definitely all of the holohedral space groups7 VJ, to V, 6 (2Di—1 to 2Di—16) based on this lattice except V, V, Vj,3 and V 6. 

Here, we will discuss the two most common types of prism, namely (i) 60°-prisms used for light dispersion and (ii) prisms used as total reflectors. 

Two types of prisms, namely 60° Cornu quartz prism and 30° Littrow prism are usually employed in commercial instruments. The latter is preferred. Simple glass prisms are used for visible range. For ultraviolet region silica, fused silica or quartz prism are used. Flourlte is used in vacuum ultraviolet range. Ionic crystalline materials are used in the infrared region. Some examples are NaCl, KBr, CsBr, and the mixed crystalline material commonly called KRS-5. 

As shown in Figure 7-20b, the IMirow prism, which permits more compact monochromator designs, is a 30° prism with a mirrored back. Refraction in this type of prism takes place twice at the same interface so that the performance characteristics are similar to those of a 60° prism in a Bunsen mount. 

A right-angle prism using total internal reflection off the uncoated hypotenuse deviates a beam by 90 , if it impinges perpendicular to one of the entrance surfaces. This type of prism is very useful for deviating high-power laser beams. 

The first requirement is a source of infrared radiation that emits all frequencies of the spectral range being studied. This polychromatic beam is analyzed by a monochromator, formerly a system of prisms, today diffraction gratings. The movement of the monochromator causes the spectrum from the source to scan across an exit slit onto the detector. This kind of spectrometer in which the range of wavelengths is swept as a function of time and monochromator movement is called the dispersive type. 

As in all Fourier transform methods in spectroscopy, the FTIR spectrometer benefits greatly from the multiplex, or Fellgett, advantage of detecting a broad band of radiation (a wide wavenumber range) all the time. By comparison, a spectrometer that disperses the radiation with a prism or diffraction grating detects, at any instant, only that narrow band of radiation that the orientation of the prism or grating allows to fall on the detector, as in the type of infrared spectrometer described in Section 3.6. 

The instrument is equipped with a prism so that the colour of the liquid under examination and that of the standard which has been selected on the disc are brought into juxtaposition, thus making it easier to compare the two. The Nesslerimeter is a similar type of instrument, made in a tall form to accommodate Nessler tubes. 

The presence of first-order reflections from all types of pyramidal planes (Table II) eliminates from consideration all space-groups based on any but the simple orthorhombic lattice r0. Of these the following are further definitely eliminated ) by the occurrence of first-order reflections from the prism planes given in Table III ... 

Figure 3. Relation between the orthorhombic structure types of CrB and UBC (projected along [100]) as well as between the tetragonal structure types of a-MoB and ThBC (projected along [100]). Boron atoms are at the centers of trigonal metal prisms, carbon atoms at the centers of MjB-Oj,. The numbers given indicate the heights in projection.

T ike metals minerals also exhibit typical crystalline structures. As an example, the structure of molybdenite is shown in Figure 1.17. It is hexagonal with six-pole symmetry and contains two molecules per unit cell. Each sulfur atom is equidistant from three molybdenum atoms and each molybdenum atom is surrounded by six sulfur atoms located at the comers of a trigonal prism. There are two types of bonds that can be established between the atoms which constitute the molybdenite crystal stmcture. They are the covalent bonds between sulfur and molybdenum atoms and the Van der Waals bonds between sulfur-sulfur atoms. The Van der Waals bond is considerably weaker than the covalent sulfur-molybdenum bond. This causes the bonds of sulfur-sulfur to cleave easily, imparting to molybdenite the property of being a dry lubricant. Molybdenite adheres to metallic surfaces with the development of a molecular bond and the friction between metallic surfaces is replaced by easy friction between two layers of sulfur atoms. 

Gold nanostructures of various sizes and morphologies were synthesized at room temperature using naturally occurring biodegradable plant surfactants [ 76]. The sizes and shapes (spherical, prisms, and hexagonal) of the synthesized nanopartides were dependent on the concentration of the gold ions and the type of plant surfactant used for preparation. 

The wall-PRISM theory has been extended to multiple site models [95], A simple example of a multiple-site model is a vinyl polymer (e.g., polypropylene) where there are three types of united-atom sites corresponding to CH2, CH, and CH3 groups. Ignoring end effects as before the PRISM equations take the form... 

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