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Homework Help: Focal length

  1. Apr 30, 2007 #1
    1. The problem statement, all variables and given/known data
    An object and a screen are separated by 20 cm. A convex lens is placed between them, 5 cm from the object. In this position it causes a sharp image of the object to form on the screen. What is the focal length of the lens?

    3. The attempt at a solution
    wouldn't it just be 5 cm since it is producing a sharp image on the screen? the distance from the object to the lense?
  2. jcsd
  3. Apr 30, 2007 #2
    looking at this more I think i need an equation. I dont have a book so can someone show me the equation im looking for?
  4. Apr 30, 2007 #3


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    Homework Helper

    You need the thin lens equation:

    [tex]\frac{1}{f} = \frac{1}{p} + \frac{1}{q}[/tex]

    where f is the focal length, p is the object distance, and q is the image distance.
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