On 12 January 2012 11:25, Dirk Kostrewa <kostr...@genzentrum.lmu.de> wrote: > 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.
I'm not a physicist either but if I look up 'coherence' in Wikipedia (not necessarily the most accurate source of information I admit!): "The most monochromatic sources are usually lasers; such high monochromaticity implies long coherence lengths (up to hundreds of meters). For example, a stabilized helium-neon laser can produce light with coherence lengths in excess of 5 m. Not all lasers are monochromatic, however (e.g. for a mode-locked Ti-sapphire laser, Δλ ≈ 2 nm - 70 nm). LEDs are characterized by Δλ ≈ 50 nm, and tungsten filament lights exhibit Δλ ≈ 600 nm, so these sources have shorter coherence times than the most monochromatic lasers." ( http://en.wikipedia.org/wiki/Coherence_%28physics%29 ). So coherence is indeed directly related to monochromaticity so there's no energy dispersion on elastic scattering. Of course X-rays from any source will also have (more or less depending on the physics of X-ray production) a characteristic Δλ which implies some degree of incoherence in the incident and therefore the scattered beams. The question though is whether or not the scattering event adds to this intrinsic incoherence. When we talk about 'coherent scattering' we mean that the degree of incoherence of the scattered beam is unchanged relative to that of the incident beam. Cheers -- Ian Cheers -- Ian
