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Homework Help: Help_How to use graph to find focal length?

  1. Jan 10, 2010 #1
    1. The problem statement, all variables and given/known data
    Rearrange the lens equation to make 1/v the subject of the equation and then write it as a linear equation. This will lead you to a possible graph for data of object distances and image distances from which you will be able to evaluate the focal length f of the convex lenses.

    2. Relevant equations
    *lens equation: 1/f=1/u+1/v

    3. The attempt at a solution
    I don't really understand what I'm supposed to do. I mean, usually we have equations like y=kx and all we do is to find the slope k. But for the equation 1/v=1/f-1/u ? I really don't get this. Should I make it into v=uf/(u-f)? I think that's the right way but I dont know if i can get f using that equation. Please HELP!
    Last edited: Jan 10, 2010
  2. jcsd
  3. Jan 10, 2010 #2


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    Welcome to PF!

    Hi linda92! Welcome to PF! :wink:

    You want a straight line,

    so instead of having v along one axis, you have 1/v instead.

    And along the other axis, you have … ? :smile:
  4. Jan 10, 2010 #3
    Re: Welcome to PF!

    Hello! Thanks :)

    The other side I made it 1/u.
    In Graphical Analysis I entered the function "f(x)=1/f-x", with x=1/u, f(x)=1/v..

    ...And it worked :)
  5. Jan 10, 2010 #4


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    :biggrin: Woohoo! :biggrin:
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