Bragg's Law

Physics: Bragg's law: 2d·sinθ = nλ (W.H. Bragg & W.L. Bragg 1913, Nobel 1915). Path difference = 2d·sinθ; constructive interference when this equals integer multiples of λ. For Cu Kα (λ=1.54 Å): silicon (111) d=3.14 Å diffracts at 2θ=28.4°; quartz (101) d=3.34 Å at 2θ=26.6°. Ewald sphere construction: diffraction occurs when a reciprocal lattice point lies on the sphere of radius 1/λ. Darwin width of a perfect crystal reflection: arcseconds (microradians). Mosaic crystal: mosaicity 0.1–1°, reflecting range broader. Laue method: white beam simultaneously satisfies Bragg for many reflections.