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Homework Help: Focal length of combined glass lens and water lens

  1. Aug 18, 2014 #1
    1. The problem statement, all variables and given/known data

    please refer to the photo,

    2. Relevant equations

    3. The attempt at a solution
    why cant I directly use the lens maker formula to get the ans ?
    my working is 1/f = ( (1.5-1.33) /1.33) x ( (1/20) + 0 ) ...finally , i get my f = -160cm.
    the radius of curvature of glass is 0

    Attached Files:

  2. jcsd
  3. Aug 18, 2014 #2


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    Strange, I get +160 cm using the lens maker formula as you write it.
    That would be for a glass lens fully submerged in water (the 1.33 for n0)

    But the exercise has a glass lens with air on one side and water on the other PLUS a water lens (concave!) with glass on one side and air on the other. In other words, a combination of TWO lenses.

    The solution manual deviously inserts a thin layer of air between the lenses so that the n0 can be set to 1 for both lenses.
  4. Aug 18, 2014 #3
    understand it now finally!
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