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Homework Help: Lens Power (diopters)

  1. Mar 29, 2010 #1
    1. The problem statement, all variables and given/known data
    A middle-aged man starts to wear eyeglasses with lenses of 2.0 D that allow him to read a book held as close as 25 cm. Several years later, he finds that he must hold a book no closer than 35 cm to read it clearly with the same glasses, so he gets new glasses.

    What is the power of the new lenses?

    2. Relevant equations
    P = 1/f (f in meters)

    3. The attempt at a solution
    P = 1/.35 = 2.9 (it was wrong)
  2. jcsd
  3. Mar 29, 2010 #2
    By your reasoning the original pair of glasses should have been 4 D (P = 1/0.25 = 4.0). Think about it some more.
  4. Mar 30, 2010 #3
    So the only thing I can come up with is .5/.35 = 1.4 (based on what you said)
  5. Mar 31, 2010 #4
    I'm still not sure what I'm doing wrong here. Help?
  6. Mar 31, 2010 #5


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    In diptres the focal length of the eye and the glasses add.

    Currently, eyeD + 2D = 1/0.35
    Wants, eyeD + xD = 1/0.25
    You are trying to find x
  7. Mar 31, 2010 #6
    ok i still don't completely understand, but I come up with 3.1 ???
  8. Mar 31, 2010 #7
    That's what I get as well but here's how I did it.

    Several years later an object placed 35 cm away from the lens focuses on his retina. Using the thin lens equation I found the retina's distance from the lens. Then I said that a new lens will focus an object at 25 cm at the retina's distance and just used the thin lens equation again.
  9. Mar 31, 2010 #8
    Ok cool. Thanks everybody!
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