Interactive Tutorial about Diffraction Basic example II: Pair of atoms 

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Now we extend our first example by adding a second atom. Again the
1D and 2D Fourier transform are shown below:
The middle image shows a one dimensional cut through the center of reciprocal space parallel to the vector between the two atoms, while The right image shows a 2dimensional section through reciprocal space. You can see that the intensity still drops off continuously with increasing scattering vector. Perpendicular to the vector between the atoms this decrease is not changed at all compared to the previous example. Parallel to the vector however the intensity distribution becomes modulated. At evenly spaced positions the intensity is high and vanishes in between, corresponding to constructive and destructive interference of the Xrays scattered by the two atoms. Compare the result for Xray and neutron scattering using the interactive simulations. 
© Th. Proffen and R.B. Neder, 2003 