1. The problem statement, all variables and given/known data We have a honeycomb lattice. There is an incident wave of 400 nm, whose wavevector has an angle of 75 degree with the surface's normal. This problem is similar to LEEDS ( low energy electron diffraction). 2. Relevant equations What is the condition of diffraction i.e. which orders achieve Von-Laue conditions ? 3. The attempt at a solution I tried to put that |G_2d|<=2*|k_s|, where G is the 2-dimension reciprocal lattice vector, and k is the length of the scattering wavevector ( |K_incidence|=|K_scattering|).