Translational symmetry in the reciprocal lattice representation

Equations (1) are the von Laue conditions, which apply to the reflection of a plane wave in a crystal. Because of eqs. (1), the momentum normal to the surface changes abruptly from hk to the negative of this value when k terminates on a face of the BZ (Bragg reflection). At a general point in the BZ the wave vector k + bm cannot be distinguished from the equivalent wave vector k, and consequently 

Imagine k increasing along a line from the zone center T to the face center at Vibm. When k = Zibm + 8, where 8 is a small increment in k normal to the face, 

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