6.
Lawrence Bragg in Cambridge
Von
Laue had proved that X-rays were waves by diffracting
them with a crystal. He had recorded his results photographically,
with bright spots showing points where many X-rays were
in phase with each other. There were a large number
of points where these spots appeared to be 'missing'.
Diffracted beams of X-rays were expected in these directions,
but didn't seem to occur. Von Laue suggested that the
X-rays must contain only certain wavelengths to account
for the missing diffracted beams.
 William
Lawrence Bragg wasn't convinced by this explanation.
In the Autumn of 1912 Lawrence Bragg had just received
his degree in Natural Sciences from Cambridge and he began
investigating Von Laue's X-ray patterns. Lawrence Bragg
thought that X-rays must be made up of a continuous
spectrum of all possible wavelengths in the same way
that white light is made of a spectrum of all the possible
colours. If this was true then the 'missing' directions
of diffraction wouldn't be due to the wavelength of
the X-rays, but due to some property of the crystal
being examined.
Lawrence
Bragg thought of each plane of atoms in a crystal as
a reflecting surface. The X-rays would hit each plane
of atoms in turn, reflecting first off the surface layer,
then the one below it, and so on. If the X-rays reflected
off all the surfaces were in phase, with their peaks
and troughs all aligned, then a very strong signal could
be measured from the reflection.
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