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Homework Help: Determining the focal length of a compound lens

  1. May 15, 2016 #1
    1. The problem statement, all variables and given/known data
    A biconvex lens, [itex]L_1[/itex], with focal length of magnitude [itex] \left|
    = 15[/itex] cm is used with a second biconvex lens, [itex]L_2[/itex], with focal length of magnitude [itex] \left|
    = 25[/itex] cm to form a compound lens system.

    If the two lenses are placed in mutual contact what is the focal length of the compound lens?

    2. Relevant equations

    [itex]\frac{1}{f_c} = \frac{1}{f_1} + \frac{1}{f_2}[/itex]

    3. The attempt at a solution
    It's just a matter of plugging into the above equation.. However do I use the the absolute values or the fact that for a biconvex lens

    f_1 > 0[/itex]


    f_2 < 0[/itex]?

    Solution given is [itex] +9.375 [/itex] cm, which suggests you use the absolute values, however I am not fully convinced as it was not specified in the course notes whether to use absolute values or not.

    Not using absolute values, I find the focal length to be [itex] +37.5 [/itex] cm
  2. jcsd
  3. May 15, 2016 #2
    Sorry, I figured it out... both are biconvex lenses therefore their focal lengths are positive. I believe I got confused with radius' of curvatures.. the solution given is the correct one.
  4. May 15, 2016 #3
    For a biconvex lens:

    R1 > 0 and R2 < 0 (Radius of curvature)

    not the focal lengths... :confused:
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