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**1. Homework Statement**

My book uses the following equation to derive the diffraction condition for electromagnetic waves scattering in a crystal lattice:

[tex]F= \int dV n(\mathbf{r}) \exp \left[i\Delta\mathbf{k}\cdot \mathbf{r} \right][/tex]

F is the scattering amplitude and n is the electron density. I just don't understand where that comes from. How does the Fourier transform relate to the diffraction? I have studied Fourier expansions in calculus and I understand how (virtually) every periodic function can be represented as a Fourier series.

**2. Homework Equations**

**3. The Attempt at a Solution**