= Electron diffraction
{wiki}
<Electron diffraction> is <interference> between scattered <Electron> <probability amplitudes>. It connects the <de Broglie wavelength> of the incident beam to the arrangement of the scattering material. In <elastic scattering> from a fixed crystal, $|\mathbf k_{\rm out}|=|\mathbf k_{\rm in}|=2\pi/\lambda$, and constructive <crystal scattering> selects <wavevector> transfers $\mathbf Q=\mathbf k_{\rm out}-\mathbf k_{\rm in}$ in the <reciprocal lattice>. The physical <momentum transfer> to the <Electron> is $\hbar\mathbf Q$ in this convention.
<Bragg's law> expresses the same condition as $2d\sin\theta_B=n\lambda$, where $d$ is the spacing of the chosen lattice planes, $n$ the <diffraction order>, and $\theta_B$ the angle to those planes. The angle between incoming and outgoing propagation directions is $\chi=2\theta_B$ for that reflection geometry. In normal-incidence backscattering, a detector's angle $\phi$ from the outward surface normal instead gives $\chi=\pi-\phi$; neither detector colatitude nor angle to the surface normal should silently replace $\theta_B$. A <finite-row electron diffraction model> uses a different spacing, the spacing between sites along a surface row, and makes that distinction explicit.
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