Elastic Bragg scattering condition
= Elastic Bragg scattering condition
For elastic scattering with $k-k'=q\in\Lambda^*$ and $|k|=|k'|=k$,
$$
2k\cdot q=|q|^2,
\qquad
|q|=2k\sin\frac\theta2.
$$
The smallest nonzero diffraction angle is determined by the shortest reciprocal-lattice vector.
This reciprocal-lattice condition is equivalent to <Bragg's law>, expressed in terms of the lattice-plane spacing and the wavelength.