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Homework Help: Convex lenses

  1. Feb 10, 2015 #1
    1. The problem statement, all variables and given/known data
    A thin double convex lens of focal length f is broken down into two halves at the axis. The two haves are then combined, as shown in the figure. What will be the focal length for the arrangements?

    http://www.meritnation.com/app/webroot/img/shared/content_ck_images/images/Untitled(375).png [Broken]
    2. Relevant equations

    3. The attempt at a solution
    For (i), I think that the answer is infinite because any incident ray will either go undeviated or will be laterally displaced - but the combination will not be able to converge it.
    Is it right?

    And, about the second arrangement - I think it must be 2f, but in my book, f/2 has been given as the answer!
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Feb 10, 2015 #2

    Quantum Defect

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    Homework Helper
    Gold Member

    Your answers look good to me. I think you even realize that f/2 is the correct answer, based upon your drawing...

    The combined lens will be more focusing -- i.e. shorter focal length. 2f is a focal length 2X as long. Can that possibly be right, given your picture?
  4. Feb 10, 2015 #3
    Ahh! I got it!
    Power is inversely related to focal length. Now, because the combination is "more" converging (more power), its focal length will be less. :headbang:
    I think it's correct now.
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