Scattering by Two Material Points

It is equal to the momentum taken by a fixed material point. In our case, modulus Ikl = Ikol (i.e. A,=Xo). This corresponds to elastic scattering because the points do not take energy from the photons and the light frequency remains unchanged. Hence, as seen from the figure the scattering wavevector amplitude is 

This is very general equation that will be used further on. From the same figure we can extract another useful relationship between the q-vector and the wave path difference A accumulated along the distance between the particles. It is just a difference of two scalar products  

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