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Adding to what James wrote: I see this as follows: Bragg's law is only a necessary but not a sufficient criterion for occurrence of a diffraction peak if viewed as a reflection. The problem imho comes from not considering the structure factor as the actual quantifier of (single photon) diffraction. Take for example the reflection condition rules for I centered cells: Sum of the three reflection indices all even. 111 333 missing but 222 is there... The problem does not arise if you treat the diffraction pattern as a probability distribution where the intensities are proportional to the (Square of) the structure factors which includes the hkls and thus automatically and necessarily the Bragg ns. Taking the simple 2-wave Bragg reflection picture verbatim does not do justice to the diffraction process (e.g. the non-coherence between incoming photons and necessity for a single photon process have been discussed here before). James has already suggested a few historic reasons. Best, BR -----Original Message----- From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of Dom Bellini Sent: Dienstag, 20. August 2013 16:49 To: [log in to unmask] Subject: Re: [ccp4bb] Dependency of theta on n/d in Bragg's law Dear Pietro, Ladd & Palmer book does explain it, just first example that springs to mind. HTH D -----Original Message----- From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of Pietro Roversi Sent: 20 August 2013 15:37 To: ccp4bb Subject: [ccp4bb] Dependency of theta on n/d in Bragg's law Dear all, I am shocked by my own ignorance, and you feel free to do the same, but do you agree with me that according to Bragg's Law a diffraction maximum at an angle theta has contributions to its intensity from planes at a spacing d for order 1, planes of spacing 2*d for order n=2, etc. etc.? In other words as the diffraction angle is a function of n/d: theta=arcsin(lambda/2 * n/d) several indices are associated with diffraction at the same angle? (I guess one could also prove the same result by a number of Ewald constructions using Ewald spheres of radius (1/n*lambda with n=1,2,3 ...) All textbooks I know on the argument neglect to mention this and in fact only n=1 is ever considered. Does anybody know a book where this trivial issue is discussed? Thanks! Ciao Pietro Sent from my Desktop Dr. Pietro Roversi Oxford University Biochemistry Department - Glycobiology Division South Parks Road Oxford OX1 3QU England - UK Tel. 0044 1865 275339 -- This e-mail and any attachments may contain confidential, copyright and or privileged material, and are for the use of the intended addressee only. If you are not the intended addressee or an authorised recipient of the addressee please notify us of receipt by returning the e-mail and do not use, copy, retain, distribute or disclose the information in or attached to the e-mail. Any opinions expressed within this e-mail are those of the individual and not necessarily of Diamond Light Source Ltd. Diamond Light Source Ltd. cannot guarantee that this e-mail or any attachments are free from viruses and we cannot accept liability for any damage which you may sustain as a result of software viruses which may be transmitted in or with the message. Diamond Light Source Limited (company no. 4375679). Registered in England and Wales with its registered office at Diamond House, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, United Kingdom