Week 4: Lecture 7, 8, Color centers

Minerals develop colors by interacting with white light when only a part of the visible spectrum is absorbed leaving the rest to emit. When this happens, the energies absorbed are removed from the emitted light which will no longer be white, but have complementary color associated with the emitted wavelengths. The observed color of a mineral thus corresponds to those wave lengths of light that are least absorbed. When a solid sample absorbs light, what we see is the sum of the remaining reflected colors that strike our eyes. If all the light energies are reflected back, the sample will appear colorless or white. However, it will appear black if all the energies are absorbed, leaving none of the visible spectrum reaching our eyes. The study of defects in ionic crystals through their interaction with electromagnetic radiation has been an important area of research for several decades. Such research has not yet directly resulted in the development of many important devices, although such developments are by no means precluded. In fact, much of the best work from a scientific point of view has been done on alkali halide crystals, which have not proved very useful technologically. The importance of alkali halide color-center research for the solid-state chemist would seem to be that in these simple model solids so much can be learned

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