Scattering by one or two charge points

When more than one charge unit is involved, interference sets in and must be taken into account through a phase difference (Fig. 5.9) between the diffracted beams. Consider two charge units, A and B, and an arbitrary scattering direction denoted by angle 0 (see Fig. 5.10). The path difference between the wave scattered by A and that scattered by B is 

The phase difference between any two points separated by vector rt is given by 2n times the path difference between the two points, divided by the wavelength X  

A further very useful relationship gives the expectation value of the exponential term in a wave, when averaged over all possible orientations in space  

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