# Talk:Bragg's law

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## Suggested merge with Bragg diffraction

At 2007-01-18T01:22:20 (UTC), Sojourner001 (talk · contribs) suggested that this article be merged with Bragg diffraction. Brian Jason Drake 04:43, 12 April 2009 (UTC)

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)

## 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)
This image is NOT correct for Bragg's Law (BL). This BL scattering formalism comes from the assumption that the planes are infinite. Therefore, one cannot break the lateral symmetry as suggested by this image. To whit, momentum transfer must be along the symmetry breaking axis that is perpendicular to the lattice planes.--131.113.92.219 (talk) 08:06, 25 October 2014 (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).

### 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.
Many years have passed, but despite a few good suggestions above, the introductory section is still quite confusing. Bragg's law is not about diffraction patterns arising from scattering from multiple atoms, but about the combined intensity of reflections from multiple parallel (possibly ideal) planes. The distance between atoms within a plane does not matter - only the distance between planes. Bragg's law tells us what happens to the reflected intensity when we change the angle of incidence. This is entirely different from the diffraction pattern that occurs from a single plane of atoms for a given fixed angle of incidence. These 2 "kinds" of diffraction are being confused with each other in the first section, and the two associated images are not very helpful. The left part of the second image (Braggs_Law.svg) is fine and shows an example of constructive Bragg diffraction, but the right part of the same image is missing the point. Destructive Bragg diffraction should be illustrated by altering both the incident and reflected angles to achieve destructive interference while keeping the 2 angles equal. Bragg's law says nothing about what happens at angles of reflection that are not equal to the angle of incidence - that topic belongs to the "Diffraction" article. Yawe (talk) 23:10, 15 April 2012 (UTC)

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)//

### 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.

You are wrong. No derivation uses AB+BC (as defined here) as the path difference, or else you wouldnt arrive at Bragg's law. Some derivations define their triangles differently (they draw a right angle from the incident point of the first beam to the intersection with the second beam. So the lines AB and BC are already less than those defined here. —Preceding unsigned comment added by 130.207.75.24 (talk) 16:31, 20 January 2011 (UTC)

## 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.
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)
Another nice quotation about that, this time from Max Perutz, who worked under Bragg at Cavendish Laboratory:
"Why did this twenty-two year old student succeed in correctly interpreting the diffraction pattern predicted and discovered by an accomplished theoretician eleven years his senior? Bragg himself modestly attributes it to a "concatenation of fortunate circumstances" but his paper soon convinces you that its success owed more to Bragg's astute powers of penetrating through the apparent complexities of physical mechanisms to their underlying simplicity.
This first paper was quickly followed by another, written in collaboration with his father, on their newly developed X-ray spectrometer, and a third, written by himself alone, solving the structure of common salt and showing how the Laue pictures of several other simple minerals could be indexed. There follows the structure of diamond, solved, as he relates, largely by his father, and the structures of fluorspar, zincblende, iron pyrites, calcite and dolomite solved by himself alone. Finally, on July 16th 1914, he communicated a paper on the structure of metallic copper. In view of this published record and the fact that for most of the relevant period the father was at Leeds and the son at Cambridge, it seems hardly believable that the scientific public tended to attribute most of the credit for these discoveries to his father, sometimes with the undertone that the son had cashed in on his father's success. The son must have suffered a good deal from these thoughtless judgements." Sir Lawrence Bragg by Dr. M. F. Perutz, at the Cavendish Laboratory site
Avihu (talk) 11:19, 17 October 2008 (UTC)

## n?

"n is an integer determined by the order given" This is insufficiently described. Order of what? n does not appear in any of the images, so, is is always 1? Where does it come from, when is it used (should all be added to the text). Adacus12 (talk) 23:52, 20 May 2010 (UTC)

## Merge from Bragg diffraction

Yesterday (March 28 2011) a user (Franergy1) merged the content from Bragg diffraction into this article. The problem with this is that there was no prior discussion on such a merge (that I know of) and also Bragg's Law is not necessarily the same as Bragg diffraction. I therefore belatedly open the merge up for discussion.Polyamorph (talk) 07:43, 29 March 2011 (UTC)

## Reciprocal Space

The whole paragraph on reciprocal space is one big non sequitur. I don't know what kind of "Bragg Experiment" the author was doing, but when doing x-ray diffraction, you don't measure lambda or d; you measure *theta*. Then you use the magical mathematic technique of division to get d. Perhaps the author got a D on his lab report in Physics 101 for reporting n as the distance between the crystal planes?? A mistake that obvious doesn't really seem worthy of discussion. So what is that paragraph about? — Preceding unsigned comment added by 72.227.106.136 (talk) 06:44, 5 February 2012 (UTC)

```     o From my non-expert research, this is important for understanding and discussing concepts related to diffraction.
o but the text is:
O ...
O scattering angle
o why is it 2 theta? (instead of theta)
o what does its introduction consist of?
O obviously, it's not the *concept* of reciprocal lattice that is the Fourier space...
O not well motivated (why is this useful?)
O I, at least, don't understand what G is (it looks like both a scalar and a space.)
O "Q-space" is:
O only referred to (not defined)
O a space of scalars??
```

--PMH232 (talk) 22:23, 24 November 2012 (UTC)

I have a couple of comments on the article:

1. Regardless of what secondary sources may say, Bragg's law was originally published in the following reference: Proceedings of Cambridge Philosophical Society, vol XVII, page 43-57. In that publication, W. L. Bragg appears as the sole author, and the paper was introduced by Sir J. J. Thomson. In addition to secondary sources, I think that this reference should be included in the section. I think that the fact that W. L. Bragg was the sole author of the manuscript should be mentioned as well, since authorship is the base for attributing scientific merit in modern research. The paper does acknowledge other scientists, but no other Bragg is mentioned there.

2. If you read the paper mentioned in 1 (there is a digitized copy available online of the whole volume), it is clearly stated that the main source of inspiration of Bragg's work was the earlier publication by von Laue. Bragg's aim is to offer a more straightforward interpretation of the X-ray diffraction experimentally observed by von Laue. In von Laue's work the traditional derivation of diffraction based on the scattering of an incident plane wave by an ordered collection of atoms is explained, showing that the authors already understood the basics of XRD. In its current form, the section gives the impression that Bragg (father and son) both discovered X-ray diffraction, while in reality von Laue's Nobel prize was awarded for demonstrating the diffraction of X-rays in crystalline solids. Bragg's contribution was extremely important, but precedents should be acknowledged. I suggest that this fact, together with the original von Laue's work, are included as well (if not in the text, as a footnote).

Anglyan (talk) 05:03, 2 April 2012 (UTC)anglyan

## In General

This whole entry assumes reflective diffraction; it would be very helpful to show how Bragg's Law applies to transmittive diffraction. --PMH232 (talk) 22:17, 24 November 2012 (UTC)

There are two pedagogical problems with the diagram BraggPlaneDiffraction.svg. First, the angles are more commonly measured from the the normal and not from the horizontal. I suggest relocating theta and emphasizing Snell's Law at the same time. Second, drawing parallel lines to represent the Bragg planes rather than dots will accommodate the valid criticism above that diffraction/scattering is different to reflection. This change will make the reader think of the correct model. I do not have the skills to change diagrams. The second criticism applies to the first two diagrams which represent atoms as dots. It might be easier to note that constructive interference occurs when d*sin(theta)=n*lambda and destructive interference when d*sin(theta)=(n+1/2)*lambda. Then you only need one modified diagram. SPG — Preceding unsigned comment added by 130.95.16.16 (talk) 05:47, 9 April 2013 (UTC)

— Preceding unsigned comment added by 130.95.16.16 (talk) 05:31, 9 April 2013 (UTC)

This article needs a lot of work. It is very hard to follow and after reading some of the talk page I am even more confused. The article is repetitive and seems contradictory in places. I am surprised it has achieved the rating it has. Kate, 3rd year biochem student — Preceding unsigned comment added by 103.9.43.156 (talk) 08:10, 12 June 2013 (UTC)

## Heuristic derivation

I mildly rewrote the tail ends of the definition section and the heuristic derivation section to be smoother, I hope. 178.38.21.137 (talk) 21:06, 9 November 2014 (UTC)

## Removed lead too long tag to article

I removed the lead too long tag to the article after cleaning up the Intro and adding an History section.

LauraIsabelleDB 16:05 20 Marsh 2015 (UTC)