Talk:Bragg's law

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The merge is not necessary, since Bragg's Law or Bragg's condition can occur in other related areas, outside Bragg's diffraction. For example in Brillouin scattering [See for Example "Brillouin Scattering in Cubic Crystals" G.B. Benedek et al, Phys. Rev. 16,2,1966] Alektzin

I disagree, for the simple reason that it took me a long time to find Bragg diffraction and I instead spent my time milling around here, which is somewhat less useful. Sojourner001 00:24, 18 January 2007 (UTC)

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[edit] Image:Loi de bragg.png

I think this image is not quite correct, since in the second diagram, the angle of incidence does not equal the angle of reflection. :) I'd've corrected that myself, but I don't have neccessary software... --Koveras 09:34, 5 November 2005 (UTC)


This image is fine. Diffraction is not like reflection. A-priori all angles are possible, but only those that give constructive interferences actually occur, as in the left image. The right image depicts the a destructive interferences, and therefore this will not be detected. --Hezy 22:50, 20 November 2005 (UTC)

The entire image is incorrect. The Bragg condition holds for a single photon, a second coherent wave in not required. Its like Young's Two Slit Experiment except more complicated.

Your assumption assumes that each "wave" represents one particle. Instead it could represent "one possible path for a particle", so, the image is not *a priori* incorrect; and it illustrates the point quite cogently. —The preceding unsigned comment was added by 137.131.130.118 (talk) 00:39, 24 January 2007 (UTC).


[edit] The Point May Have Been Missed
  • I partialy agree with this anonymous fellow, above. For Bragg's law assumes that diffraction from crystal planes can be ragarded as the reflection of radiation off paralell planes. What Hezy says is very correct, "Diffraction is not like reflection. A-priori all angles are possible...", but in regard to a discussion about Bragg's Law, I'm sure diffraction is regarded the same as reflection. Bragg's Law is a model for diffraction, a simplification, that is not likley to hold in every case.
I do not however know that the Bragg condition holds only for a single photon, im sure it applies to waves equally. Indeed the wave needs to be monochromatic, but yes, there does not need to be a second coherent wave.

I make several recomendations:

  1. This article should concentrate on Bragg's Law first, not theory that should be learned after comming to grips with Bragg's Law.
  2. The image does need to be removed, as i believe that it does not show how Bragg's Law works in terms of lattice, or scattering, planes.
  3. There should be a derivation to complement the Law so that a someone new to diffraction can understand it. At the moment i think only someone who has a prior understanding of the theory can follow it, and it should be made more accessible to lowwer levels.
  4. I propose that is article can be re-written to accomidate the above

Honourable Crammer 04:22, 1 November 2006 (UTC)


Seconded - Thomson scattering occurs in all directions, but we have a whole separate article discussing that. Moving the geometric derivation to just after the historical section I think would make it more accessible. This goes double if we want to keep a separate article dealing with just Bragg's Law, with most of the crystallography and whatnot under Bragg Diffraction.
The Rayleigh image should then be moved to a 'more exactly microscopic' section following this. The loi_de_bragg image in my opinion needlessly complicates the article - if we show constructive interference, destructive interference leading to no measured reflection is implied; also, it digresses naturally into a discussion of finite crystallite size effects, which would be definitely off-topic.
Eldereft 16:30, 22 February 2007 (UTC)
What? No please don't remove this image, it's the only one that actually makes sense. I was taking a crystallography class, and I didn't quite understand the explanation of Braggs' law presented to me, but when I saw this image (and the succeeding one) I was able to understand it. As for the succeeding image, however, someone should really change it, since the 45 degree angles really make it look like AC' = BC, which results in the wrong equation all together.

I also think that the change Whiner01 made to the Angstrom unit was unnecessary. Although the unit of measurement is not in SI units, an angstrom Å, is still a unit of measurment commonly refered to in journals, text books, internet and class rooms when refering to distances in the order of atom spacings. But, at the same time, wikipedia is about making information more accessible to everyone, so perhaps it was warrented for that reason? i dont know. I shall think more clearly before placing a comment in the future. Honourable Crammer 13:12, 1 November 2006 (UTC)//


[edit] Derivation

The derivation presented in the article is different from the widely-accepted derivation. Any reason for this? The widely accepted derivation uses AB + BC as the path difference. There is no mention of AC', AC, or any of those. To my eye, I cannot even see how a path difference of something less than AB + BC can resolve to the same as AB + BC.

[edit] Credit

As far as I know Bragg's law was derived solely by Sir W.L. Bragg and his father had nothing to do with it. Look for example in The Nobel Prize in Physics 1915 Presentation Speech. Avihu (talk) 19:10, 17 March 2008 (UTC)

Should I accept the silence as a token for approval to change the article? Avihu (talk) 15:42, 19 March 2008 (UTC)

I don't think this is clear cut. Some sources say it was him alone but many many texts, university websites attribute it to both. Some attribute it to him but there appears no consensus. The authorship on the 1912 paper should really be the definitive source - anyone have a copy ? The best I can find is P.47 of Light Is a Messenger: The Life and Science of William Lawrence Bragg, Graeme K. Hunter, 2004 —with a clear statement that William Bragg alone derived the law — although this is contradicted in other texts with attribution to both - Peripitus (Talk) 20:46, 19 March 2008 (UTC)
What about The Nobel Prize in Physics 1915 Presentation Speech? To my mind that should be enough, but I can add two more , “Great Solid State Physicists of the 20th Century", Julio Antonio Gonzalo, Carmen Aragó López (Most of the book is accessible in Google Books and LAWRENCE BRAGG’S ROLE IN THE DEVELOPMENT OF SOUND-RANGING IN WORLD WAR I (look at page 276). Avihu (talk) 21:07, 19 March 2008 (UTC)
I agree there are good sources for sole derivation (and really don't care what the article says). There are, however, some sources for dual derivation. Academic American Encyclopedia P.435 and others. I think the best way to proceed may be
In physics, Bragg's law is the result of experiments into the diffraction of X-rays or neutrons off crystal surfaces at certain angles, derived by physicist Sir W.L. Bragg in 1912, and first presented on 11 November 1912 to the Cambridge Philosophical Society.
add a reference here
Later sources attribute the discovery to W.L Bragg and his father W.H. Bragg - Peripitus (Talk) 02:01, 20 March 2008 (UTC)
That is a very good suggestion. Avihu (talk) 05:16, 20 March 2008 (UTC)
Well I did the change. I must confess that giving the Nobel prize site and his biographies the same credibility as an entry in an Encyclopedia (which I have to admit I never heard of) looks to me as statement that the truth is relative and we should reflect all views and let the readers decide. There are some cases, and this is one of them, that the truth stands out. The reason some sources attribute the discovery of the law to both the father and son is quite clear. William Lawrence Bragg was only 22 when he discovered the law and he talked about it with his father who was the first to publish an article about the law in Nature magazine giving credit to "My son". There is a nice section about that in LAWRENCE BRAGG’S ROLE IN THE DEVELOPMENT OF SOUND-RANGING IN WORLD WAR I:
"The prize also helped to ease Bragg’s feelings towards his father, which had been shaken during their collaboration, as we know from his own testimony in later life. Father had reported some of their joint work in two letters to Nature that gave credit to ‘my son’ for the equation but did not even give his name. In 1913 father had been invited to the Second Solvay Conference, along with Einstein, Laue, Rutherford and other luminaries of international physics. In his presentation father described his son’s results, giving him the credit. Einstein and other distinguished attendees sent Lawrence a postcard of congratulations. Lawrence felt that his father had been unjust in not permitting him to describe his idea first. Father had acted unthinkingly, not maliciously, and subsequently went out of his way to make amends. Still the incident was ‘.remembered 60 years later with pain’. Surely this contributed to Bragg’s lifelong vigilance to see that credit was always given where due—a key element in his successes as a scientific administrator."
Avihu (talk) 09:44, 21 March 2008 (UTC)