Optical elements for X-rays are based on the principle of grazing incidence reflection. Reflection, absorption, and transmission are expressed through the complex index of refraction, which can be written as n=1-δ-iβ where δ describes the phase change and β accounts for the absorption. The optical constants δ and β are functions of the wavelength or the photon energy (Fig. 6.1). For X-rays, the real part of n, 1 — δ, is slightly less than unity for matter whereas it is exactly unity in vacuum. X-rays propagating in vacuum, therefore, undergo total external reflection when incident below the critical grazing angle α{int} with cos α{int} = 1 — δ according to Snell’s law. Because of the nonvanishing value of β the reflection is actually not total for α ≤ c, but is less than unity, and X-rays are reflected at angles even larger than the critical angle α{int} [1]. There are also characteristic absorption edges resulting from the specific atomic structure of each element.
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© 2008 Springer-Verlag Berlin Heidelberg
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Friedrich, P. (2008). Wolter Optics. In: Trümper, J., Hasinger, G. (eds) The Universe in X-Rays. Astronomy and Astrophysics Library. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-34412-4_6
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