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Homework Help: KE of Electrons to give same theta as light in double slit diffraction

  1. Mar 7, 2009 #1
    1. The problem statement, all variables and given/known data

    Coherent light is passed through two narrow slits whose separation is d. The second-order bright fringe in the interference pattern is located at an angle of theta.

    If electrons are used instead of light, what must the kinetic energy of the electrons be if they are to produce an interference pattern for which the second-order maximum is also at theta?

    2. Relevant equations

    No relevant equations givven

    3. The attempt at a solution

    Well, I started by saying that the wavelength of the electrons will be the same as the wavelength of light, and thought of using h=p lambda and then E=hf.

    However I then confused myself because for h=p lamba I need the momentum and for that I need the speed. E=hf will give me kinetic energy wouldn't it? Can I please have some hints and pointers as to what direction I should be thinking in?
  2. jcsd
  3. Mar 7, 2009 #2

    Doc Al

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    Staff: Mentor

    Good. Use that to find the momentum of the electrons.
    That would apply for photons, not electrons.

    How does momentum relate to kinetic energy?
  4. Mar 7, 2009 #3
    OK. I used Ek = p^2/(2m) and h=p lambda

    to give Ek= h^2/(lambda^2*2m)

    now do I use n lmbda = dsin theta?
  5. Mar 7, 2009 #4
    done it! thanks guys!
  6. Mar 7, 2009 #5

    Doc Al

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    Staff: Mentor

    Sounds good to me.
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