a question about Bragg's experiment

Discussion in 'Physics' started by logicman112, Oct 19, 2010.

  1. logicman112

    Thread Starter Active Member

    Dec 27, 2008
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    2*d sin(teta) = n * lambda
    This is the formula of Bragg. How can we measure 'n'?
    An important fact which no physic book wants to talk about!!
    'n' is the order of the reflected beam, what does it mean? How we can measure it?
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    If none of 'em talks how did u know abt it?
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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  4. Wendy

    Moderator

    Mar 24, 2008
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    Do you mean this?

    Bragg's Law

    *************

    Curses! Beenthere and moved on!
     
  5. logicman112

    Thread Starter Active Member

    Dec 27, 2008
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    Last edited: Oct 25, 2010
  6. Papabravo

    Expert

    Feb 24, 2006
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    Your mistake is the expectation that there is a single solution -- there isn't. The statement that "n is an integer" just means that there may be multiple solutions depending on the wavelength of the light and the geometry of the crystal lattice. Why are you being so obstreperous anyway?
     
  7. DeminJanu

    New Member

    Oct 26, 2010
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    as stated above, there are many solutions.
    every time 2*d*sin(theta) equals an integer multiple of the wavelength, you see a bright spot. ie. you see a whole set of bright spots, equally spaced apart.

    similar to this:
    http://en.wikipedia.org/wiki/File:Single_slit_and_double_slit2.jpg
    (slightly different math, but same idea of coherent interference from regularly spaced sources/reflection)
     
  8. logicman112

    Thread Starter Active Member

    Dec 27, 2008
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    I thought Bragg's experiment can measure the wavelength. So it can not!!
    Because there are two variables unknown, n and the wavelength!!

    So why the following extract from a famous physic book states that William Bragg invented this theory to measure wavelength?
    ---------------------------------------------------------------------------------------
    In 1913, William Bragg and his son Lawrence, proposed a simple method of
    measuring the wavelengths of X-rays by means of their ‘reflection’ from crystal planes.
    Crystals comprise atoms (or ions) that are arranged in a spatial lattice, such that they....
    ---------------------------------------------------------------------------------------
     
    Last edited: Oct 31, 2010
  9. logicman112

    Thread Starter Active Member

    Dec 27, 2008
    69
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    Please add your comments then I will let you know something which has never been mentioned in physic books. How 'n' can be measured in Bragg's experiment logically?
     
  10. Markd77

    Senior Member

    Sep 7, 2009
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    Am I missing something? I'd have thought you increase the angle from zero until you see a patch of X-rays. That means n=1. The next patch is n=2, etc.
    I imagine you see X-rays at angle = 0 because they go through the sample - ignore that.
     
    logicman112 likes this.
  11. logicman112

    Thread Starter Active Member

    Dec 27, 2008
    69
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    Mark, you are right.
    We see the first bright spot in the minimum path difference and the minimum path difference happens when n=1.
     
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