Bragglagen

Bragglagen, known in English as Bragg's law, is the fundamental relation in X-ray crystallography that describes the condition under which X-rays diffracted by crystal planes interfere constructively. It relates the wavelength λ of the incident X-ray, the interplanar spacing d, and the angle θ between the incident beam and the crystal planes. Constructive interference from successive planes occurs when nλ = 2d sin θ, with n a positive integer.

Origins and recognition: The law was derived in 1913 by William Henry Bragg and his son William Lawrence Bragg at the University of Leeds, to explain X-ray diffraction patterns from crystals. They were awarded the Nobel Prize in Physics in 1915 for their work on crystal structure analysis.

Applications: Bragg's law enables the determination of lattice spacings and crystal structures. By measuring diffraction angles for a known X-ray wavelength, one computes d-values for various lattice planes. It is central to powder diffraction as well as single-crystal diffraction and underpins much of materials science, chemistry, and physics.

Limitations: The law assumes monochromatic, coherent radiation and a well-ordered crystal with parallel planes. It is a kinematic approximation that does not account for multiple scattering, extinction, or significant absorption; real materials may require corrections for mosaicity, strain, or defects. Nevertheless, it remains widely applicable for determining crystal structures within its domain of validity.