Understanding Bragg's Law in X-Ray Diffraction

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Discussion Overview

The discussion revolves around Bragg's Law in the context of X-ray diffraction, specifically addressing the assumptions made regarding the parallelism of reflected rays and their implications for interference and superposition. Participants explore the theoretical underpinnings and practical applications of the law in experimental methods such as rotating crystal and powder photography.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the assumption that reflected rays can be treated as parallel, arguing that they must meet for superposition to occur, which seems inconsistent with the derivation of Bragg's Law.
  • Another participant suggests that parallel incident waves reflecting from parallel planes should result in parallel reflected rays, questioning the need for a lens to achieve interference.
  • A different viewpoint is presented, stating that when an electromagnetic wave strikes an atom, it emits a spherical wave front, implying that the emitted rays from two atoms, while parallel, do not meet for interference without additional optical elements.
  • One participant counters that the reflected rays are only approximately parallel and that this approximation is sufficient because the point of convergence is far away.
  • A participant acknowledges the similarity of assumptions made in Bragg's Law to those in Fraunhofer diffraction, indicating a recognition of the underlying principles.

Areas of Agreement / Disagreement

Participants express differing views on the treatment of reflected rays in relation to their parallelism and the necessity of lenses for interference, indicating that multiple competing perspectives remain without a clear consensus.

Contextual Notes

The discussion highlights limitations in the assumptions regarding the parallelism of rays and the conditions under which Bragg's Law is applied, without resolving these uncertainties.

manofphysics
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We all know, Bragg's law , n\lambda=2dsin\theta
where 2dsin\theta=path difference.
In the derivation of the path difference we take the two incident rays to be parallel which is perfect BUT we take reflected rays also to be parallel.How can this be since the reflected rays have to MEET for superposition to take place.
This is the same as in FRAUNHOFER diffraction, where we CAN take parallel rays due to the distance being effectively infinity or due to the use of LENS.
But we are not using a lens nor is the distance supposed to be very large(infinity) in the experimental methods of Xray diffraction ( rotating crystal, powder photograph etc.).
So, how is it that this formula works?
 

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I'm missing something here.

You have parallel incident waves reflecting from parallel planes...why wouldn't the reflected rays also be parallel?

Or are you trying to treat the wavefront as a single point rather than a plane?
 
Last edited:
when an EM wave strikes an atom, a spherical wave front is emitted. ie waves are emitted in ALL directions . Now we are considering the emitted rays by two atoms which are parallel,
But the parallel rays will not meet for interference, we need to use a lens...
(I am using an analogy of diffraction in optics here. as both are almost similar, I think)
 
The rays do meet because the rays are not exactly parallel. They are only approximately parallel. And the approxmation is good enough because the place where they meet is "far" away.
 
So, I guess it is the same assumption that we take in Fraunhofer diffraction.
Thanks for the reply ,Edgardo.
 

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