User talk:PMH232

Bragg's Law for Transmission

Hello,

I guess you are the one who contacted me on fr:.

I left the business 6 years ago, so I'm don't pretend to be as accurate as I was, but AFAIK, there no difference between reflection and transmission. The only difference is the range of the deviation angle 2θ:

it is not possible to measure low 2θ angles in reflection because the spot spreads on the sample, so you have more aberrations, and the spot needs to be small in order not to get out of the sample (thus low intensity);
it is not possible to measure high 2θ angles in transmission because the thickness that is seen by the beam becomes very important, thus too much absorption.

Appart from this, I don't know anything about opals…

Hope this helps.

cdang|write me 07:41, 27 November 2012 (UTC)[reply]

Sorry, I used the Brags law for X-ray diffraction. X-ray are stopped by a few μm of materials, therefore the imitation I mentioned. In case of transparent material, this is of course not the case.

However, what you have to remember is that nodal planes are only in the imagination of the analyst. What you really have is diffusion by the nodes of the crystal lattice, the diffusing waves interfering. Brag's law is only a simple way to express the diffraction condition.

So, you don't really have reflection, or transmission, you just have diffusion, and reflection is just a convenient way to express things. It is also valid for transmission, and nobody uses negative values for 2θ as it is a deviation.

cdang|write me 19:57, 28 November 2012 (UTC)[reply]

Well, I didn't look at web pages as it was one of my main skill fex years ago (studied it, had plenty of paper references). I don't think you need much math to understand this, as long as you don't try to look at the electromagnetic side of it. Try to look at crystallography pages, with description of the atom positions as vectors — for basic skills, look at Euclidean vector and Euclidean space.

An important point is the symmetries of the crystal, which determines the symmetry of the diffracted pattern; sources frequently refere to space groups, but this rather complicated notion is not important to understand things, you can see it as a classification of the lattices. Look a Bravais lattice.

So, if you understand the notion of coordinates of an atom in a cell, and the notion a normal vector, then you can understand what the Miller indices are. This is an important notion as the diffaction spots are indexed by Miller indices. Diffraction spots correspond to constructive interferences of waves diffracted by atoms which situated on neighbouring planes — remember these are imaginary plane, this is only a way to says that the distance between the atoms, d, is linked to the diffraction angle 2θ. This relationship is… the Bragg's law.

Depending on what you are interested in — understand how diffraction occurs, or understanding the arranngement of the diffraction spots —, focus on interfrence or on crystallography.

I will be happy to answer some more specific questions.

cdang|write me 10:25, 29 November 2012 (UTC)[reply]

Your recent edits

Hello and welcome to Wikipedia. When you add content to talk pages and Wikipedia pages that have open discussion (but never when editing articles), please be sure to sign your posts. There are two ways to do this. Either:

Add four tildes ( ~~~~ ) at the end of your comment; or
With the cursor positioned at the end of your comment, click on the signature button ( or ) located above the edit window.

This will automatically insert a signature with your username or IP address and the time you posted the comment. This information is necessary to allow other editors to easily see who wrote what and when.

Thank you. --SineBot (talk) 13:07, 28 August 2013 (UTC)[reply]