For each unit cell there is an incident beam that is diffracted and emanates from the cell. The conditions for diffraction call for constructive interference only at points that an integral number of the wavelength. Single crystal versus powder diffraction. In the case of powder diffraction the sample is ground to a fine powder. When the beam strikes the powder, all possible crystallographic orientations are presented and therefore all possible diffraction occurs.
Most powder diffraction is conducted using parafocusing methods.
This figure shows the geometrical principles involved in a parafocusing diffractometer. X-rays diverge from a point source 2. G1, G2, G3 on the goniometer circle. In this case the radius of the focusing circle is constant. In parafocusing systems the detector pivots about the sample to maintain a constant distance between the sample and detector.