If you work with thin films and have access to an X-ray diffractometer with grazing incidence capabilities, choosing the right incidence angle is not just a detail, it determines whether you are actually probing your film or the substrate underneath. In this post, I walk through the theory and calculation of the X-ray penetration depth in Grazing Incidence X-ray Diffraction (GIXRD), using SnO$_2$ as a worked example. Why does the incidence angle matter? In a conventional Bragg-Brentano ($\theta$–$2\theta$) XRD measurement, the X-ray beam penetrates several micrometers into the sample. For a thin film of tens or hundreds of nanometers, most…