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Homework Help: Focal Points of a Thick lens

  1. Feb 8, 2012 #1
    1. The problem statement, all variables and given/known data

    A thick lens has an index of refraction of 1.560, thickness of 3.0cm, and radii of curvature of R1= -4.50cm, R2= -3.60cm.

    Calculate the positions of the focal points (relative to the vertices).

    2. Relevant equations


    [itex]ffl = \frac{f1(t + f2)}{t-(f1 + f2)}[/itex]

    3. The attempt at a solution

    I assume I need to first focal length for each lens to plug into the front focal length and back focal length formulas, meaning s' = ∞ so for L1:

    f1 = [itex]\frac{R}{n'-n}=\frac{-4.50cm}{1.560-1.000}[/itex] = -8.04cm

    Do I have the right idea? If so, will I need to swap the indexes of refraction when doing the opposite side of the lens, so that light is coming from the left side?
  2. jcsd
  3. Feb 9, 2012 #2
    These might help?

    "For the case of a lens of thickness d in air, and surfaces with radii of curvature R1 and R2, the effective focal length f is given by:"

    From half way down the following page,


    Worked examples here,


    Found via,



    Good luck!
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