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Bragg, Sir (William) Lawrence

ray crystallography crystals father

(1890–1971) British physicist: founder with W H Bragg of X-ray crystallography.

Born in Adelaide, W L Bragg studied mathematics there and at Cambridge, and in 1910 moved his interest to physics. Like his father, he was attracted by observation that X-rays could be diffracted by crystals. Bragg showed that the condition for diffraction by a crystal with lattice planes (layers of atoms) d apart, for X-rays of wavelength ? and angle of incidence ?, is that n ? =2 d sin? (Bragg’s Law) where n is an integer. The atomic layers of a crystal acted as mirrors, reflecting X-rays, with interference resulting from reflections at different layers when the angle of incidence met the above condition. Using an X-ray goniometer made by the father (who had taken instruction in instrument-making in Adelaide) the pair were able to measure X-ray wavelengths and then to measure d , the inter-atomic distance, in crystals of diamond, copper, sulphur and salts such as KCl (which they found contained only ions and no molecules). Previously, crystallography had been concerned with the angles at the exterior of crystals; now X-ray crystallography could study their atomic interior.

At 25, Lawrence Bragg was the youngest Nobel prizewinner, sharing the prize in 1915 with his father. In 1919 he became professor at Manchester and in 1938 at Cambridge. He developed methods whereby X-ray diffraction by crystals (giving on a photographic plate or film a pattern of spots whose position and intensity could be measured) can be used to determine electron density within the crystal and therefore the position of the atoms. Modern metallurgy, crystallography and molecular biophysics owe much to his methods and to those of his co-workers in Cambridge. Like his father he became director of the Royal Institution (in 1954), did much to popularize science and was knighted.

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