I'm not a physicist - but isn't (in)coherence also used to describe the
property of sources of electromagnetic waves with constant wavelength?
For instance, an incoherent sodium vapour light source (only looking at
one emission band) compared to a coherent Laser, or the incoherent
emission from a conventional X-ray source or an X-ray undulator compared
to a Free-electron-X-ray-Laser? If yes, then we could describe
diffraction from a crystal in a similar way by treating the crystal as a
"light-source", both with coherent and incoherent scattering from the
well-ordered and disordered parts, respectively, without any need to
change the wavelength. In this analogy, the ordered part would have the
coherence of a Laser, whereas the disordered part would have the
incoherence of a vapour lamp.
Best regards,
Dirk.
Am 12.01.12 11:57, schrieb Ian Tickle:
On 12 January 2012 10:33, Dirk Kostrewa<kostr...@genzentrum.lmu.de> wrote:
My understanding of coherence is a constant phase relation between waves.
Correct. For a perfect crystal all the unit cells are identical so
they scatter in phase
and this gives rise to the interference effect we see as Bragg spots,
as you say arising
from a constant phase relation in specific directions. For a disordered
crystal the unit cells are not the same: this destroys the
interference effect but there's
still a constant phase relation in any specified direction so it's
still coherent.
Of course, this breaks down for inelastic scattering, but (in)coherence can
also be described without any change in wavelength.
That's not the definition of incoherence used by the physicists. Of
course you're
free to redefine it but I think that just confuses everyone.
Cheers
-- IAn
--
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Dirk Kostrewa
Gene Center Munich
Department of Biochemistry
Ludwig-Maximilians-Universität München
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