I also find that a bit confusing. If you want to think about it in terms of quantum approach (i.e., individual photons), the following points are worth keeping in mind:
(1) All of this is elastic scattering, falling within the first-order Born approximation. What this means physically is a photon is elastically scattered from a spherically-symmetric potential — effectively a billiard ball. It neither absorbs or emits energy. Scattering from a single point yields a spherical wave, but that wave in the single-photon quantum limit has to be interpreted as a probability amplitude, not an electric field vector. (2) A single photon only interferes with itself. (3) A photon scattering from a given atom within a macromolecule in the crystal feels the scattering contribution only from the other identical, crystallographic symmetry-related atoms in the other asymmetric units of the crystal that are exposed to the X-ray beam, and all of these contribute equally to the diffraction pattern. (The diffraction pattern is in essence the spherically scattered wave sampled by the reciprocal lattice). (4) This is physically the same problem as a single photon scattering from a multiple (effectively infinite) slit diffraction grating. Classically, we would assume the photon only goes through one of the slits, but quantum-mechanically, we have to consider all of them in order to correctly predict the diffraction pattern. (My guess is what Bernhard wrote is a way of describing this non-classical behavior). BTW that really has to be the ickiest user name alias in the history of the CCP4 BB. William G. Scott http://scottlab.ucsc.edu/~wgscott > On May 21, 2015, at 6:43 PM, Murpholino Peligro <murpholi...@gmail.com> wrote: > > Hello Everybody! > I was trying to make some sense from Bernhard Rupp's book page 251. > > I will copy the relevant part... > > When photons travel through a crystal, either of two things can happen: (i) > nothing, which happens over 99% of the time; (ii) the electric field vector > induces oscillations in all the electrons coherently within the photon's > coherence length ranging from a few 1000 Angstroms for X-ray emission lines > to several microns for modern synchrotron sources. At this point, the photon > ceases to exist, and we can imagine that the electrons themselves emanate > virtual waves, which constructively overlap in certain directions, and > interfere destructively in others. The scattered photon then appears again in > some direction, with the probability of that appearance proportional to the > amplitude of the combined, resultant scattered wave in that particular > direction.......The sum of all scattering events of independent, single > photons then generates the diffraction pattern. > > I underlined the problematic parts... > > can anyone shed some light on this ..or point me in the right direction? > > > Thanks in advance > >
