Leeds University Library

The Bragg law

WLB announced his interpretation in a paper, The Diffraction of Short Electromagnetic Rays by a Crystal(4), read to the Cambridge Philosophical Society on 11th November 1912 and subsequently printed in the Proceedings in 1913. An Abstract was published in Nature (5) (December 5th 1912) which was followed a week later (December 12th) by a further communication entitled the Specular Reflection of X-rays(6) in which, following a suggestion of CTR Wilson made during a discussion of his earlier paper, he demonstrates the reflection of X-rays from the cleavage planes in mica.

In the Cambridge Philosophical Society Paper we have for the first time the Bragg law in embryonic form:

nλ = 2dcosθ

Where θ is the angle of incidence of the x-rays measured to the normal of the reflecting planes and is complement of the angle θ (measured to the planes) first used in the joint paper read to the Royal Society on 17 April 191311

How was it that a 22 year old student succeeded where all the 'best brains' of Europe had failed? Genius certainly, but perhaps a genius whose mind was not encumbered with irrelevant and complicated theory, and which drew heavily on simple optical diffraction theory.

There is a story recounted by R W James, a later close colleague of WLB. At a meeting of English and German students in Leipzig in 1931 it did not go unnoticed by the German delegates that, in comparison with their own education, there was a distinct lack of rigorous training in mathematics and physics among the English. "Tell me" confided one of James's German colleagues: "how does Bragg discover things? He doesn't know anything!"