Bragg-type scattering effect

Cholesteric liquid crystals, e.g., those of cholesteroylnonaoate (see Sec. 3.2), produce a Bragg-type scattering, which depends on temperature and angles of incidence and observation. Either total reflection or total transmission of circular polarized light is observed, which effect provides the basis of the dark-bright liquid crystal display in the Schadt-Helfrich cell (Fig. 3.5.3) as well as color reflection. 

The displacements of atoms may take several forms (Figure 13.2). These have been described by Jack Dunitz, Verner Schomaker, and Kenneth N. Trueblood as follows The perfectly ordered crystal would have every atom firmly fixed to its own perfectly defined site in each unit cell for the entire period of observation. There are, however, various types of disorder from unit cell to unit cell. If the atom jumps to a different site, that is one kind of disorder [a mixture of static and dynamic disorder ] if it moves to and fro, that is another kind of disorder [ dynamic disorder] if it is forever in one site in a certain unit cell and in a different site in another cell, that is still another kind [static disorder]. Each of these types of vibrations, displacements, and disorder has somewhat similar effects on the intensities of Bragg reflections the effect they have in common is that they reduce these intensities by an amount that increases with increasing scattering angle, 26, as shown in Figure 13.1. 

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