> Maybe some physicist out there can give a more rigorous explanation.
More likely s/he will say, The answer to your question can be found in equation 2.8 of the text available at:
http://www.amazon.com/s/ref=nb_sb_ss_i_1_22?url=search-alias%3Daps&field-keywords=the+optical+principles+of+the+diffraction+of+x-rays&sprefix=the+optical+principles%2Caps%2C135
As for scattering vs diffraction, I think scattering is the general term, diffraction is specifically when scattering from different points interferes constructively and destructively, giving rise to a "diffraction pattern".
On 05/24/2016 05:48 PM, Alex Lee wrote:
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> On Tue, May 24, 2016 at 2:45 PM, Edward A. Berry <[log in to unmask] <mailto:[log in to unmask]>> wrote:
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> Good questions! You might add a third fork and ask how electrons are scattered by matter, e.g. in electron diffraction or cryo-EM single particle analysis.
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> >From my limited understanding, the way charged particles (such as electons or nuclei) scatter electromagnetic radiation (like x-rays) is something like this:
> Electromagnetic radiation results in an oscillating electric field at the particle, which results in an oscillating force if the particle is charged. This force accelerates the particle, in proportion to its charge and inversely with its mass. The oscillating charged particle emits radiation (in all directions?) like a radio antenna. (the accelerating particle is then affected by the magnetic field in some way that I do not pretend to understand). Although the charge of the electron is smaller than that of most nuclei, its mass is vastly smaller, so that charge oscillates through a much greater distance, absorbing and re-emitting radiation much more strongly.
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> This is a very classical explanation (and incomplete at that) and ignores consideration of wave-particle duality and the fact that a single photon is scattered, even though it passes by the particle presumably in a time much less than one period of its frequency (how many wavelengths long is a photon? Coherence length?). But it gives a nice intuitive explanation of why electrons are responsible for the vast majority of x-ray scattering by matter. Maybe some physicist out there can give a more rigorous explanation.
> eab
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> On 05/24/2016 05:00 PM, Alex Lee wrote:
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> Dear CCP4BB members,
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> I'm a biologist who is using X-ray crystallography to study protein structures. I read books and internet posts for layman to understand X-ray crystallography (I have limited physics background).
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> Often times I read that " X-rays interact primarily with the electron cloud surrounding each atom. Neutrons interact with nucleus of atoms". I do not understand why X-rays could not interact with nucleus of atoms. I read that electrons even do not have a volume, and weighed much less than nucleus, but it seems it contribute mainly to X-ray diffraction.
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> Another question is that some posts talks about electrons "diffract" X-rays, and some talks about electrons "scatter" X-rays. Even with the help of Wikipedia, I could not tell what is the difference between "diffract" X-rays and "scatter" X-rays.
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> Thanks ahead for any illustrations on this.
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