The concept of employing extreme oblique incidence (‘off-plane mount’) is of importance not only for neutron optics but also for X-rays in designing spectrometers for space applications, for instance, as well as for extreme UV light at grazing incidence. As the next step, starting again from normal incidence, we tilt the grating around its grating vector by an angle \(\zeta\) to obtain oblique incidence, and measure the angular dependence also for this more complicated situation. To vary the angle of incidence, the grating is rotated in steps by angles \(\theta\) about an axis perpendicular to the latter plane (see Fig. First, we describe measurements of the angular dependence of the diffracted intensities for the simple and usual case of in-plane diffraction, i.e., for incoming and outgoing beams lying in the same plane. We deploy a planar, one-dimensional unslanted grating whose diffraction properties cannot be described by either of the two extreme cases outlined above. Here, we elucidate one of these general cases experimentally: oblique incidence on a (holographic) phase grating exhibiting diffraction in the so-called intermediate diffraction regime that is in between the Raman-Nath regime for optically thin gratings and the Bragg regime for optically thick gratings. Theories for almost any conceivable configuration other than the two mentioned above are treated in the literature (see, e.g., ), but remain widely unknown to most non-specialists. It is clear that the above two cases are two rather simple extremes of more general, complicated situations in diffraction physics. Instead, the condition under which constructive interference occurs and a sharp diffraction maximum can be observed is given by Bragg’s law. In this case, normal incidence does not lead to any diffraction. Second, Bragg diffraction from thick gratings is introduced within the context of determining crystal structures. The numerous observable diffraction maxima that exhibit little dependence on the angle of incidence are usually explained in terms of multi-wave interference with the diffraction angles being governed by the grating equation (see, for instance, ). A standard approach to diffraction phenomena is based on the following two simple cases: first, diffracting light from optically thin gratings at normal incidence is considered.
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