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Homework Help: Optics question: What two positions of a lens gives a sharp image?

  1. Aug 22, 2008 #1
    1. A lamp and a screen are 1m apart and a +4.5 D thin lens is mounted between them. Where must the lens be placed in order to produce a sharp image on the screen, and what will be the magnification?

    2. Gauss Law: L' = L + F

    3. I attempted this question by using the Gauss Law but I just don't know how to find the two possible positions of the lens to give a sharp image. I thought that a sharp image might be formed at F or F' but it doesn't work. Can someone please show me how to do this problem and explain it :)
  2. jcsd
  3. Aug 22, 2008 #2


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    Why doesn't it work? What do you know about "F" ?
  4. Aug 22, 2008 #3


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    Welcome to PF!

    Hi LeeLoo! Welcome to PF! :smile:

    I don't think that's the right law …

    shouldn't it have 1/… ? :wink:
  5. Aug 22, 2008 #4
    Oh, yeah, the law is actually n/l + F = n'/l' but i thought people could abbreviate it like that. anyway, yay, i know how to do the question now!

    1/l' + 1/l = F
    l' +l =1m
    l' = 1 - l

    so sub that in and get two answers for 'l' which are the lens positions :D

    and magnification= l'/l so you get m = -2 or -1/2

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