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Homework Help: Two Lense Problem

  1. Mar 21, 2005 #1
    A converging lens of focal length 20 cm is seperated by 50 cm from a converging lens of focal length 5 cm. (A) Find the final positiom of the image of an object placed 40 cm in front of the first lens. (B) If the height of the object is 2 cm, what is the height of the final image. (C) Determine the image position of an object placed at 5 cm in front of the two lens in contact.

    I am having some issues with two lense problems.

    A)In A if the image is 40 cm infront of the first lens it is 10 from the second. The image will come up somewhere inbetween the two? Im assuming I need the equation 1/f=1/d0 + 1/di, but do I use it twice or what.

    B) This one isnt that bad, once I find A this shouldnt be an issue.

    C) Part C is sort of just like A, once I can figure out how to solve A this one shouldnt be that bad.
  2. jcsd
  3. Mar 21, 2005 #2


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    Staff: Mentor

    Yes, you have to use it twice. First, pretend the second lens isn't there, and find the position of the image that the first lens produces. Second, pretend that the first lens isn't there any more, put the second lens back where it was, and pretend that the object is located at the image position that you calculated in the first step. In other words, the image produced by the first lens becomes the object for the second lens. It's a good idea to make a scale drawing to keep track of where everything is, and see how the distances fit together.
  4. Mar 21, 2005 #3
    Alright thanks, that makes sense
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