Comer cubes

In a transmission insertion probe (see Figure 4.2) light makes only a single pass through the sample gap. In these probes either the fiber is forced into a U bend at the end of the probe, or comer-cube reflectors must be used to turn the light path 180°. Again, the smallest practical gap is 0.5mm (0.5-nun optical path). 

There are two types of optical communication systems passive reflective systems and active-steered laser systems [War 01, War 05], A passive reflective system, such as a comer-cube retroreflector (CCR), consists of three mutually orthogonal mirrors that form the comer of a cube. Light entering the cube bounces off the mirrors and is reflected back to the sender. By electrostatically actuating the bottom mirror, the orthogonalty can be disturbed and the reflection is no longer returned to the sender. 

Figure 10.29. Schematic diagram of the second Cologne terahertz spectrometer [59]. Details of the comer cube mixer, the mixer-multiplier and the far-infrared laser are not included.

The open-path experiments were carried out using a transmitter-retro-reflector-receiver configuration, where the laser radiation was transmitted into the atmosphere and returned back to the detector along a coaxial delivery/retum path. A comer-cube retro-reflector was placed at the required distances between 220 m and 2900 m from the transmitter/receiver site... 

Another way to free the mirror carriage from the high precision requirement is to use retroreflectors instead of flat mirrors. Then a small tilt of the reflector does not matter. The original instrument of Fellgett used comer cubes. High precision requirements are shifted from the motor design to that of the reflector, where they are easier to handle, but are still not trivial. The Cassini spacecraft (to arrive at Saturn in July 2004) and the Mars Express spacecraft (to arrive at Mars in December... 

Factory Mutual Corner Test. This is a large-scale comer test used to test building products (18—20). The test rig consists of three sides of a cube. The two walls are 15.24 and 11.58 m by 7.62 m tall. The ceiling is 9.14 x 15.24 m. The product to be tested is mounted on the walls and ceilings ia a manner consistent with the iatended use. The fire source is a 340 kg stack of wood pallets located ia the corner. In order to pass the test, no flame can propagate to any extremity of the walls or ceiling. The Factory Mutual flammabiHty apparatus is proposed to replace this test for certain appHcations (21). 

Imagine, now, a solid held together by such little springs, linking atoms between two planes within the material as shown in Fig. 6.1. For simplicity we shall put atoms at the comers of cubes of side Tq. To be correct, of course, we should draw out the atoms in the positions dictated by the crystal structure of a particular material, but we shall not be too far out in our calculations by making our simplifying assumption - and it makes drawing the physical situation considerably easier ... 

Schematic representation of defect clusters in Fei- jO. The normal NaCl-type structure (a) has Fe (small open circles) and O (large dark circles) at alternate comers of the cube. In the 4 1 cluster (h), four octahedral Fe" sites are left vacant and an Fe" ion (grey) occupies the cube centre, thus being tetrahedrally coordinated by the 40. In (c) a more extended 13 4 cluster is shown in which, again, all anion sites are occupied but the 13 octahedral Fe sites are vacant and four Fe occupy a tetrahedral array of cube centres.

Simple cubic cell (SC). This is a cube that consists of eight atoms whose centers are located at the comers of the cell Atoms at adjacent corners of the cube touch one another. 

Face-centered cubic cell (FCC). Here there is an atom at each comer of the cube and one in the center of each of the six faces of the cube. In this structure, atoms at the comers of the cube do not touch one another they are forced slightly apart Instead, contact occurs along a face diagonal The atom at the center of each face touches atoms at opposite comers of the face. 

The structures of LiCl and NaCl are typical of all the alkali halides (Group 1 cation, Group 17 anion) except those of cesium. Because of the large size of the Cs+ ion, CsCl crystallizes in a quite different structure. Here, each Cs+ ion is located at the center of a simple cube outlined by Cl- ions. The Cs+ ion at the center touches all the Cl- ions at the comers the Cl- ions do not touch each other. As you can see, each Cs+ ion is surrounded by eight Cl- ions, and each Cl- ion is surrounded by eight Cs+ ions. 

LiCl. the Cl- ions aie in contact with each other, forming a face-centered cubic lattice. In NaCl, the Cl- ions are forced slightly apart by the larger Na+ ions. In CsCl. the large Cs+ ion at the center touches the Cl ions at each comer of the cube. 

Significant figure A meaningful digit in a measured quantity, 9,20-2 lq ambiguity in, 10 in inverse logarithms, 645-647 in logarithms, 645-647 Silicate lattices, 243 Silicon, 242-243 Silver, 540-541 Silver chloride, 433,443-444 Simple cubic cell (SC) A unit cell in which there are atoms at each comer of a cube, 246... 

Fig. 1. The structure of gas hydrates containing a hydrogen-bonded framework of 46 water molecules. Twenty molecules, arranged at the comers of a pentagonal dodecahedron, form a hydrogen-bonded complex about the comers of the unit cube, and another 20 form a similar complex, differently oriented, about the centre of the cube. In addition there are six hydrogen-bonded water molecules, one of which is shown in the bottom face of the cube. In the proposed structure for water additional water molecules, not forming hydrogen bonds, occupy the centres of the dodecahedra, and...

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