Physics: 1915

1) Bragg, Sir William Henry (1862-1942), was a British physicist. He and his son, Sir William Lawrence Bragg, shared the 1915 Nobel Prize in physics for their use of X rays to study the structure of crystals. By analyzing how the paths of X rays change as the rays pass through crystals, the two scientists determined the detailed arrangement of atoms within crystals. Their technique has become a fundamental part of molecular biology.

William Henry Bragg was born near Wigton, in what is now the county of Cumbria, and studied at Cambridge University in England. He served as a professor at the universities of Adelaide (Australia), Leeds, and London. Bragg later became director of the Royal Institution in London.

Contributor: Bruce R. Wheaton, Ph.D., Director, Inventory of Sources for History of Twentieth-Century Physics.

2) Bragg, Sir William Lawrence (1890-1971), a British physicist, used X rays to discover the atomic structure of crystals and large molecules. Bragg, known as Lawrence, was only 25 years old when he shared the 1915 Nobel Prize for physics with his father, William Henry Bragg, for their services in the analysis of crystal structure by means of X rays.

In 1912, Lawrence Bragg began researching X rays and crystal structure with his father, a distinguished physicist at Leeds University, in England. The Braggs shone X rays onto crystals and photographed the rays that were reflected. The atoms in a crystal are arranged in a regular geometrical pattern called a lattice, with spacings of about a ten-millionth of a centimeter. The wavelengths of visible light are hundreds of times greater than this, so no visible-light microscope can show the crystal lattice. X rays, however, have very much shorter wavelengths than visible light and can reveal detail on the atomic scale.

When X rays are reflected from a crystal lattice, the change in direction results in some rays being delayed in relation to others. (In a similar way, if two racing cars travelling abreast suddenly have to turn to the right, the one on the left will fall behind.) The amount of delay depends on the angle at which the X rays are reflected. Depending on the amount of delay, the waves may reinforce each other, giving a bright reflected ray, or may cancel each other out, weakening or even extinguishing the rays. A photograph of the rays shows a geometrical pattern of spots, formed by the reinforced rays striking the film. Complicated mathematical analysis allowed the Braggs to "decode" the pattern to reconstruct the positions in three dimensions of the crystal's atoms. The two men succeeded in determining the structure of diamond.

Lawrence Bragg was professor of physics at the University of Manchester from 1919 to 1937. From 1938 to 1954, he was Cavendish Professor of experimental physics at Cambridge University, where he directed the Cavendish Laboratory. Bragg was able to study the structure of protein molecules and other giant molecules produced in living things by forming them into crystalline solids so that they would diffract X rays. Therefore, he was able to provide crucial evidence to biochemists Francis Crick and James Watson when they were struggling at the Cavendish to discover the structure of the DNA molecule, the substance of which most genes are made. Bragg was knighted in 1941. From 1954 to 1965, he directed the Royal Institution in London, which is largely concerned with spreading public understanding of science.

Bragg was born in Adelaide, Australia, where his father was professor of mathematics at the university. He took a mathematics degree with first-class honors at Adelaide University, at the age of 18. He then went to England and entered Trinity College, Cambridge University, studying natural sciences and again gaining first-class honors in 1912.