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Moving an object and screen to find the focal length of a lens

  1. May 12, 2014 #1
    1. The problem statement, all variables and given/known data
    A convex lens is used to project the image of an object on a screen with a magnification m1 . Leaving the lens fixed, the object is moved along the optical axis
    by a distance Δs and the screen is moved until the image is located again. If the new
    value of the magnification is m2 , Show that the focal lenghth [itex]\frac{Δs}{\frac{1}{m_{2}}-\frac{1}{m_{2}}}[/itex]


    2. Relevant equations
    s'/s=m
    1/s+1/s'=1/f
    Where s is the position of the object and s' is that of the image/screen

    3. The attempt at a solution
    I used s1=s2+Δs and attempted to rearrange but got nowhere really.. any suggestions?
     
  2. jcsd
  3. May 12, 2014 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Please show your work then, so we can see what went wrong.
     
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