The Braggs: Father and Son change the World

By Laura Boness

William Lawrence Bragg was the youngest person to ever become a Nobel Laureate – he was just 25 when he and his father jointly received the prize in 1915 for their work on X-rays and analysing crystal structures.

He was born in Adelaide in 1890, where his father William Henry Bragg was a professor at the University of Adelaide. His first exposure to X-rays took place at the age of 5, when William Henry built a X-ray machine after the German physicist Wilhelm Conrad Roentgen announced that he had discovered the rays.

The machine came in handy not long after when young William Lawrence broke his arm and the machine was used to diagnose the extent of his injuries. This was the first recorded surgical use of X-rays in Australia.

Bragg was acknowledged as a bright student. He enrolled at the University of Adelaide at the age of 15 and graduated three years later. Soon afterwards, his father accepted the Chair of Physics at Leeds University and took the family back to England. William Lawrence entered Trinity College, Cambridge, in 1909 and studied mathematics and physics before going to work at the Cavendish Laboratory.

In 1912 he joined his family for a summer holiday and found that his father was excited over a new scientific paper by Max Theodor Felix von Laue, who had discovered that when X-rays passed through a crystal, they bent or diffract. Scientists were debating at the time whether X-rays were particles or waves, and this piece of evidence suggested these had to be waves. Bragg’s father, however, believed they were particles.

When Bragg returned to Cambridge, he kept thinking about this and decided there were pieces missing from Laue’s theory about diffraction. He thought that since the atoms of a crystal are arranged in pattern, the reflected X-rays could reveal the arrangement of the atoms within the crystal.

He devised a mathematical equation to study the structure of a variety of crystals, looking at the relationship between the wavelength of the X-ray, the angle at which it enters the crystal, and the distance between the crystal’s layers. This can be used to determine the three-dimensional arrangement of atoms inside crystals and became known as Bragg’s Law.

William Lawrence was teased by others for disproving his father’s own theory about X-rays, but he had actually worked on this with Bragg senior and showed that X-rays existed as waves and also as particles. Together they showed that salt crystals (sodium chloride), didn’t have any molecules of NaCl, but was made up of sodium and chloride arranged in a geometric pattern.

During this work, he and his father also perfected the X-ray spectrometer, which they used to conduct their X-ray crystallography experiments. Bragg was elected to a Fellowship at Trinity College in 1914, but the First World War interrupted his work.

Bragg was commissioned as a 2nd Lieutenant and began working on sound ranging methods for locating enemy guns 1915. While he was on duty in France later that year, he found out that he and his father had jointly received the Nobel Prize for their work in the analysis of crystal structure by means of X-rays.

The general public did not give them equal credit for their work – everyone assumed that Bragg senior had made the discoveries. He in turn tried to point out his son’s ideas in their work, but much of it was in joint papers, which made things more difficult. After the war, they agreed to end their collaboration and stick to separate areas of X-ray crystallography.

Bragg junior married and became Langworthy Professor of Physics at the Victoria University, Manchester, before looking at protein chemistry. He later worked on improving sonar and, as Resident Professor at the Royal Institution in London, became a popular science communicator, devising lectures that showed scientific experiments to school children in the 1950s and 60s.

By the time he had retired in 1966, he had witnessed X-ray crystallography go from revealing the structures of simple crystals to being used on molecules containing thousands of atoms. He died five years later, aged 81.

Did you know?
When Bragg was a boy, he discovered a new species of cuttlefish – it is named Sepia Braggii after him.