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Final image from two lenses

  1. Aug 4, 2012 #1
    1. The problem statement, all variables and given/known data
    6pmeyv.jpg

    S is a lighted object, height=1.
    what is the final image that is created by the system?
    2. Relevant equations



    3. The attempt at a solution
    i just dont understand how do i find the location of a virtual image created by a negative lens when the rays converge to a point beyond its focal point

    thnks for helpers.
     
  2. jcsd
  3. Aug 4, 2012 #2

    Doc Al

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    Staff: Mentor

    Why can't you just use the thin lens formula?
     
  4. Aug 4, 2012 #3
    you mean this?:
    1/f=1/U+1/V

    i used this for the lens 1, and found that V, image distance, is 12, which means it is located 6 to the negative's right, 2 after the focal point.

    So how can i use the equation for the second lens? U is supposed to be an object, not converging rays.
     
  5. Aug 4, 2012 #4

    Doc Al

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    Staff: Mentor

    All good.
    The image from the first lens becomes the object for the second. But since it's past the second lens, it must be treated as a virtual object. Just use the usual sign convention: If an object before the lens has a positive object distance, one after the lens would have a negative object distance.
     
  6. Aug 5, 2012 #5
    oh... you right.
    i dont know, for some reason i didnt think it would work.

    but can you explain why is that? is there a simple proof to it?
     
  7. Aug 5, 2012 #6

    Doc Al

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    Staff: Mentor

    I suggest that you review the derivation of the thin lens formula.
     
  8. Aug 5, 2012 #7
    ok, thanks for help
     
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