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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<[log in to unmask]> 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 -- ******************************************************* Dirk Kostrewa Gene Center Munich Department of Biochemistry Ludwig-Maximilians-Universität München Feodor-Lynen-Str. 25 D-81377 Munich Germany Phone: +49-89-2180-76845 Fax: +49-89-2180-76999 E-mail: [log in to unmask] WWW: www.genzentrum.lmu.de *******************************************************