(Top) A small deformation of the lattice planes in a perfect crystal has a wave-guiding effect on an x-ray wave packet, effectively translating it by several mm through the crystal ( is a reciprocal lattice vector). (Bottom) A more detailed picture of how the curved lattice planes translate the x-ray “ mode” that propagates through the crystal. The purple lines indicate the lattice planes ( denotes the displacement as a function of space) and the blue lines denote the phase of the propagating mode. As the mode curves, the amplitudes of its constituent and waves changes , with the amplitude of increasing with respect to . Since the Poynting vector is parallel to , the exiting wave is mainly along .