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Calculating power of lens?

  1. May 6, 2016 #1
    1. The problem statement, all variables and given/known data
    The contact lens prescribed for a mildly farsighted person is 1.25 D, and the person has a near point of 29.0 cm. What is the power in D of the tear layer between the cornea and the lens if the correction is ideal, taking the tear layer into account? Assume that a normal human has a near point of 25.0 cm.
    2. Relevant equations
    P = 1/do + 1/di = 1/f
     
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  3. May 7, 2016 #2

    Simon Bridge

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    The tear+contact must make a compound lens with the power needed to correct eyesight with the stated near-point.
    How do the powers of lenses combine?
     
  4. May 7, 2016 #3
    The two lenses would bee a double convex (in the eye) and the contact would be a convex, right?
    But I'm confused about how to set up the problem.
    One would be negative and one would be positive, right?
     
  5. May 7, 2016 #4

    Simon Bridge

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    Go back to basics:
    When you are given the nearpoint, and you want to know what power of lense will correct that eyesight - how do you work that out?
    When you have two lenses one after the other with P1 and P1 ... how do you find the overall power P of the combined lens?

    If you do not answer questions I cannot help you.
    Note: the shape of the lens is irrelevant.
     
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