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Homework Help: Converging lens (focal L)

  1. Aug 4, 2009 #1
    1. The problem statement, all variables and given/known data
    The figure illustrates qualitatively a converging lens focusing monochromatic rays from A to B. The vertical and horizontal scales of the figure are not the same. The top ray leaves A at an angle of 2.38 deg above the optic axis and leaves B at an angle of 1.25 deg below the optic axis. The total distance from A to B is 75.3 cm.


    Calculate the focal length of the lens for that light.

    2. Relevant equations

    1/d0 + 1/di = 1/f

    3. The attempt at a solution

    I'm not even sure where to begin when I'm given the angles. I think 75.3 cm = d0 + di
    but I'm not sure where to go from there
  2. jcsd
  3. Aug 4, 2009 #2


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    Homework Helper

    Hi NikkiNik! :smile:

    This is a geometry exercise …

    you have two triangles, you know the lengths a+b and h, and you know a/h and b/h. :wink:
  4. Aug 4, 2009 #3


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    Homework Helper

    If AOP is the axis of the lens, and AP and BP are the incident and refracted rays, then OP/do = tan 2.38 and BP /di = tan 1.25.
    Or do*tan2.38 = di*tan1.25
    And do + di is given.
    Find do and di and hence focal length of the lens.
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