Everyone knows that Sodium Chloride was the first crystal structure to be determined, and that its solution, along with the other alkali halides was greatly assisted by the ideas of Pope and Barlow. But how, exactly, did the Bragg Equation get invented?
Eighty four years after the event none of the details of that crucial
advance in crystallography are as clear as we would like, but it seems that
what happenned was something like this:
Both the Braggs - father (WH) and son (WL) - were fascinated by the question, Were X-rays really waves, not particles? WH, who had been in Leeds already for three years (as Cavendish Professor of Physics) had the service of an excellent workshop; X-rays were available in his laboratory, so the experiments of Laue, Friedrich and Knipping were easily repeated. Meantime, WL, who had newly graduated in Maths and Physics at Cambridge, was a research student in Cambridge, under J J Thompson; but he joined his father whenever possible, in the vacations. On holiday, in the summer of 1912, they began discussing Laue's discovery; soon after, they started their own experiments. Back in Cambridge after the vacation, WL studied Laue's photographs further... and it was there that he then had his brain wave.
Young (22), independent, full of ideas, and with his University Physics
lectures still fresh in his mind, WL remembered C.T.R.Wilson's lecture
course on optics, particularly CTR'S treatment of optical diffraction by a
grating. CTR had shown that a 'form-less pulse' (today, we would say, some
white radiation) is split up by a grating into its component wavelengths,
each emerging in its own geometrically determined direction; for a line
grating the directions could be specified by equations of the type, nlambda
= aSin(theta). With these ideas in his mind he also recalled the well-known
Huygens construction for the reflection of waves by the points of a plane
surface. As soon as reflection was thought of, WL found that it explained
the Laue pictures at once. (He especially liked its explanation of the
elliptical shape of the Laue spots.) He then checked his idea experimentally
with a piece of mica (it was CRT'S suggestion to use something like that)
since mica must have well-marked planes of atoms, because it cleaves so
well. Sure enough, the mica behaved just like a mirror. WL took his
photographic plate, still wet from the fixer solution, down to JJ's room to show
him, and JJ was quite excited by it. ,..So, with reflection now as the basic
concept, the Bragg equation followed almost automatically:
As for WH, in Leeds, he enthusiastically embraced the wave theory as a
result of WL's insight. The ionisation spectrometer constructed under his
direction (in that excellent workshop) was explicitly based on the
reflection principle. With that fine in- strument as their source of
experimental data (far better than Laue photos) WH and WL jointly solved the
diamond structure, and WL went on to publish the now-classic series of
papers on the alkali halides, ZnS, CaF2, Calcite, and Pyrite. That was
1912-14. Structural Crystallography had begun.
John Robertson
University of Leeds
February 1997
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