• the reciprocal lattice construction gives all the possible diffracted beams, and

• the Ewald sphere shows which of these are excited. (By excited we mean which planes give rise to diffracted beams.)

In structures with non-primitive cells, scattering from one atom in the cell can interfere with scattering from another to reduce or increase the intensity of diffraction. Click on the animation opposite to illustrate this. The unit cell of a body-centred orthorhombic crystal is shown. Identical atoms are present at each lattice point. Since the corner atoms are shared by eight cells, the unit cell can be reduced to an atom at the origin and one at the centre.