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Q: Bragg's Law is a necessary but not sufficient condition for diffraction by real crystals. It specifies when diffraction will occur for unit cells having atoms positioned only at cell corners. However, atoms situated at other sites (e.g., face and interior positions in FCC or BCC) act as extra scattering centers, which can produce out-of-phase scattering at certain Bragg angles. The net result is the absence of some diffracted beams that, according to Eq, should be present. For example, for the BCC crystal structure, h + k + l must be even if diffraction is to occur, whereas for FCC, h, k, and l must all be either odd or even. Use this information to determine the Miller indices for the first five reflections that are present for a single atom BCC and FCC unit cell. The first reflection is defined to be the one closest to 2θ = 0. (Contributed by Brian Grady)