2. Surface layers: passage layers and roughness

In the case of X-rays, Fresnel's formula can only calculate reflected intensity at angles greater than the critical angle. What's more, this Fresnel formula can only account for intensity reflected specularly, i.e. in the direction exactly symmetrical to the incident light. This excludes light scattered outside the symmetrical direction. However, any surface defect, such as scratches or compositional inhomogeneity, behaves like a diffraction grating, sending energy away from the direction of specular reflection. This is why, as a first approximation, the simplest and most frequently used surface model is that of the plane diopter: an ideal case assuming the existence of perfectly flat surfaces separating two perfectly homogeneous media, whatever the scale considered. In this model, the variation in electron density at the transition between the two media is assumed to be perfectly abrupt,...