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Bragg's Law question

  1. Feb 8, 2013 #1
    I'm preparing for a midterm exam and this is one of the practise questions I'm having a bit of trouble with.

    1. The problem statement, all variables and given/known data

    We have access to X-ray and electrons for the incident beam.
    [itex] \theta [/itex] is the inner angle between the sample and the incident beam.

    Spacing in sample = d =10^-10 m
    [itex] \lambda [/itex] = 10^-10 m

    a) What values of [itex] \theta [/itex] will we measure X-rays at a detector, that is, when will there be constructive interference (assume elastic collision)?

    b) If we use electrons instead of X-rays, at which energy should they be to detect the electrons at the same angle as the X-rays (assume elastic collision)?

    2. Relevant equations

    Bragg's Law:
    [itex]2 d sin(\theta) = m \lambda[/itex]
    E = hf
    p = h / [itex] \lambda [/itex]
    Compton effect

    3. The attempt at a solution
    For part (a) what I did was basically use:

    [itex]sin(\theta) = (1 \lambda) / (2d)[/itex]
    [itex]sin(\theta) = (2 \lambda) / (2d)[/itex]

    Since [itex]sin(\theta) = (2 \lambda) / (2d)[/itex] gives 90 degrees, my answer is that the only angle is 30 degrees, that is, m = 1.

    For part (b) I'm not really sure how to approach this question. Is this just the classical model of an elastic collision?

  2. jcsd
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