3.- When HRXRD measurements are made in a crystalline system composed of multiple layers like the one illustrated in the attached Figure, it is common to observe a phenomenon of interference in the spectrum through the appearance of the so-called "Pendellsung fringes". Why does this phenomenon occur? What information can you get from the presence of the Pendellsung fringes? In the graph shown what do the units on the x-axis mean? Fig. 2. [2 curves recorded tor samples A and B around (00U4 reflection. Best adjustment of experimental data of sample A was shown in the same figure (solid line). Pendellsung fringes prove a good structural quality of the active layers