The diffraction pattern from a crystal depends on the arrangement of atoms in the crystal. In (a) the scattered waves from atoms 1 and 2 have opposite phases and cancel out, so the scattered beam is weak along that direction. In (b) the scattered waves from atoms 3 and 4 combine to give a strong scattered beam.

A simulation of a two‐dimensional crystal and its diffraction pattern. Note that the lattice repeat in the crystal gives rise to a pattern of spots in the diffraction pattern, and that the lattice directions in the crystal are orthogonal to the lattice directions in the diffraction pattern.

Each reflection in the diffraction pattern can be described as a wave with a certain magnitude and phase. The magnitude determines the size of the wave (a), and the phase determines where the peaks occur (b).

MIR allows estimation of phases when heavy atoms are introduced in known positions. F_p represents the total scattering from the native crystal, shifted to the origin of the crystal coordinates. F_h represents the additional scattering from a heavy atom located at H. If the addition of the heavy atom leads to stronger scattering along some direction, the phase of F_p must match that of F_h (a). If the addition of the heavy atom leads to weaker scattering, the phase of F_p must be opposite to that of F_h (b).