The main reason why the instruments for visible optics have not been available for x-rays is the limitation by the available optical constants. Every material becomes absorbing for wavelengths shorter than 1100 Ã . This absorption decreases again gradually toward shorter wavelengths, but at the same time the refractive index approaches a value close to 1. This chapter defines the various parameters used to characterize the optical parameters of a material and describes their general behavior.
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