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Homework Help: Lens-makers equation

  1. Dec 3, 2017 #1
    1. The problem statement, all variables and given/known data
    The lens-makers' equation applies to a lens immersed in a liquid if n in the equation is replaced by n2/n1. Here n2 refers to the index of refraction of the lens material and n1 is that of the medium surrounding the lens.
    (a) A certain lens has focal length 92.6 cm in the air and index of refraction 1.55. Find its focal length in water.
    (b) A certain mirror has focal length 92.6 cm in the air. Find its focal length in water.

    2. Relevant equations
    1/f = (n-1)(1/R1 - 1/R2)

    3. The attempt at a solution
    I'm not really sure how to set this up. Think it looks something like this f2/f1 =(n1-1)/(n2-1) index of refraction of water 1.33 plugged in the given variables and got 55.56 which was wrong.

    Not sure what direction to go with this equation.

    Any help would be greatly appreciated.
  2. jcsd
  3. Dec 3, 2017 #2


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    That does not result from replacing n with n1/n2 in 1/f = (n-1)(1/R1 - 1/R2), as instructed.
  4. Dec 3, 2017 #3
    thanks! I figured it out earlier! Thanks for the reply though
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