In this chapter we deal with the notion of the reciprocal space and with the geometry of X-ray scattering experiments. We introduce the angles describing the orientation of the incident and scattered wave vectors with respect to a measured sample. Running an experiment, we change the orientation of the incident and exit wave vectors by rotating the sample and the detector on the goniometers. The reciprocal space is explored in scans related to the goniometer movements. We provide useful formulae for the transition between the angular and the reciprocal space coordinates.
We plot the intensity scattered by the sample versus the scattering vector, which is the difference of the scattered and incidence wave vectors. This gives us a reciprocal space map of the scattered intensity.
We use the Ewald construction as a very convenient tool to represent the scattering conditions in the reciprocal space by tracing the wave vectors involved in the scattering processes. It also shows how the reflection geometry of the experiment limits the accessible region of the reciprocal space that can be explored in coplanar scattering geometry by a radiation with a given wavelength.