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Homework Help: Velocity of a Helium-atom in a double slit experiment

  1. Jul 29, 2016 #1
    1. The problem statement, all variables and given/known data
    Hi, I want to apologize for any grammar errors in advance since english is not my first language. But i hope it is good enough such that the question is clear:
    I want to calculate the velocity of a Helium-atom after it scattered on a double slit. The following information are given:
    1) the distance of the first intensity maximum from the axis of the detector screen
    2) distance d between both slits
    3) distance from the double slit and the detector screen

    2. Relevant equations
    what is the velocity of the helium-atom? and what velocity would an electron need to have to reach the same seperation of its intensity maximum?

    3. The attempt at a solution
    1) first I calculated the condition for constructive interference: d*sin(phi)=n*lamba, n integer.
    2) from the distance to the screen L, and the distance of the first maximum, say l, we get phi: tan(phi)=l/L
    3) from the first step we get lambda, since n=1, right?.
    And here i am stuck.

    Thank you.
  2. jcsd
  3. Jul 29, 2016 #2


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    Hello Laguna, :welcome:

    Apparently you need one or more relevant equations to proceed. SInce there are none under '2. relevant equations', you will now have to search your toolbox to find some :smile:

    Also: in your attempt at solution you might show your steps in more detail; if something goes wrong, PF helper can put it right....
  4. Jul 29, 2016 #3
    Your difficulty might come from not having all the relevant equations. Think about how the velocity of a particle is related to its wavelength via an important equation.
  5. Jul 30, 2016 #4
    thank you for the warm welcome :-)

    I can use the de-Broglie relation lamba=h/p, p the momentum. then i get the speed, right? :-)
    I am sorry, I missread '2. relevant equations' with '2. relevant questions' when I was writing the post.
  6. Jul 30, 2016 #5


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    Good start. You want to check if you need anything relativistic and then you can link ##\lambda## and ##p## (or fist link them and then check that ##v<<c##)
  7. Jul 31, 2016 #6
    thank you. i didnt even condsider relativity.
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