Re: [ccp4bb] question about powder diffraction

[Boaz Shaanan]

I thought (I think I was told that way early during my PhD studies) that reflexion/reflection is a matter of British/American spelling. In fact Merriam-Webster Dictionary says just that: Definition of REFLEXION chiefly British variant of reflection   and the American Heritage and Oxford dictionaries agree on that too.  Boaz   Boaz Shaanan, Ph.D.                                         Dept. of Life Sciences                                      Ben-Gurion University of the Negev                          Beer-Sheva 84105                                            Israel                                                                                                                  E-mail: bshaa...@bgu.ac.il Phone: 972-8-647-2220  Skype: boaz.shaanan                  Fax:   972-8-647-2992 or 972-8-646-1710                          From: CCP4 bulletin board [CCP4BB@JISCMAIL.AC.UK] on behalf of Ian Tickle [ianj...@gmail.com] Sent: Wednesday, July 09, 2014 12:19 AM To: CCP4BB@JISCMAIL.AC.UK Subject: Re: [ccp4bb] question about powder diffraction Yes, the way I like to think of it as a double condition, the reflection‐in‐a‐mirror condition *plus* the special condition imposed by Bragg’s Law. This is why I often prefer the unfashionable spelling “reflexion”. -- Ian ◎ Me too.  Actually "reflexion" (but the verb is "reflect") is the original correct spelling (from Latin reflectere & reflexio); apparently at some point in its history it became misspelt due to a false analogy with "correct" & "correction" (Latin corrigere & correctio). Now back to the science!  It's important to understand that a "powder" is not amorphous which would indeed give a continuous pattern: it's a bunch of micro-crystals in random orientations.  Therefore a powder diffraction pattern is a single crystal pattern averaged over all orientations.  Rotating the crystal does not change the Bragg angles of the spots, however it does change their angular positions so each diffracted beam is smeared out over conical surface.  Each of these cones then projects as a circle on a flat area detector (of course in powder diffraction one would use a linear detector since it's not necessary to measure a complete circle). Cheers -- Ian