Crystals as Diffraction Gratings
When Max von Laue proposed in 1912 that crystals could diffract X-rays, he revealed two things simultaneously: X-rays are electromagnetic waves with wavelengths comparable to atomic spacings, and crystals are periodic arrangements of atoms. The experiment, performed by Friedrich and Knipping, launched the era of crystallography and earned Laue the 1914 Nobel Prize.
Bragg's Elegant Law
William and Lawrence Bragg simplified Laue's formalism by treating diffraction as reflection from parallel crystal planes. Constructive interference occurs when the extra path length (2d sin θ) equals a whole number of wavelengths. This simple condition — nλ = 2d sin θ — connects the observable angle to the invisible lattice spacing, enabling direct measurement of atomic-scale distances from macroscopic diffraction patterns.
From Peaks to Structure
A powder diffraction pattern — intensity versus 2θ — encodes the crystal's symmetry and lattice parameters. Peak positions reveal d-spacings (via Bragg's law), peak intensities encode atomic positions and types, and peak widths indicate crystallite size and strain. The simulation generates diffraction patterns as you adjust lattice spacing and X-ray wavelength, showing how Bragg peaks shift and split.
Impact on Science and Technology
XRD has determined the structures of over a million crystals. Watson and Crick's DNA double helix relied on Rosalind Franklin's X-ray diffraction photographs. Semiconductor fabrication uses XRD for quality control. Pharmaceutical companies verify drug polymorphs — different crystal structures of the same molecule that can have dramatically different bioavailability. From geology to nanotechnology, XRD remains indispensable.