Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Reciprocal lattice

  1. Feb 7, 2013 #1
    I got a bunch of questions about reciprocal lattice, I start with this one:

    In an x-ray experiment:

    For one specific orientation of your incident beam on your real lattice, only a portion of the points of your reciprocal lattice will become visible as your diffraction pattern right?

    See my picture


    For one incident beam, only the parallel planes are involved in your diffraction and so only the reciprocal lattice points that represent these set of parallel planes will becoem visible on your diffraction pattern

    If you want to make other reciprocal points points visible on your diffraction pattern, u have to change the orientation of your incident beam so that another set of parallel planes is involved in diffraction

    So in first picture, your blue beam will get the reciprocal points that representing the green parallel planes visible on your diffraction pattern.

    Second picture, the same blue beam does nothing on the purple planes, so the reciprocal points that represents the purple planes will not be visible on the diffraction pattern.

    Is this correct?
    Last edited: Feb 7, 2013
  2. jcsd
  3. Feb 7, 2013 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    It might help with the Forum formatting if you reduce the size of your image to no more than 800 pixels by 600 pixels. There's no reason for it to be THIS big.

  4. Feb 7, 2013 #3
    Ok, like this? But now my picture is unsharp.
    Last edited: Feb 7, 2013
  5. Feb 7, 2013 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Remove the image, and upload a new, resized version.

  6. Feb 8, 2013 #5
    Dr. Pill,

    your images are in real space, not reciprocal space.

    It is perfectly possible to align a single crystal such that two or even three sets of lattice planes fulfull the Bragg condition simultaneously.

    For the two-beam case this can be done by aligning one Bragg peak (corresponding to the reciprocal space vector Q), and by then rotating the crystal about the vector Q until you excite a second reflection (=changing the azimuthal angle). To get 3 simultaneous reflections you also need to tune the photon energy just right.

    The effect is known by several names including "Umweganregung", "Renninger effect" and "Multiple beam diffracton". It was first described in 1935.

    The following links give a bit more informaiton:

    P. P. Ewald, Rev. Mod. Phys. v37, pp46 (1965)
    This is an excellent review article about the early theory of x-ray diffraction, written by one of the founding fathers. Highly recommended if you are interested in x-rays.

    Renninger, Z. Phys v. 106 pp 141 (1937) in German

    Description of a computer code for calculating Umwegs.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook