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Homework Help: Necessary power for reading glasses

  1. Nov 10, 2011 #1
    1. The problem statement, all variables and given/known data

    A person struggles to read by holding a book at arm's length, a distance of 45 cm away (near point). What power of reading glasses should be prescribed for him, assuming they will be placed 2.0 cm from the eye and he wants to read at the normal near point of 25 cm?

    2. Relevant equations

    P=1/f (where f is in meters)

    1/f=1/do+1/di where do=distance to object and di=distance to image

    3. The attempt at a solution

    This seemed simple so I must be missing a detail. We want the object 25 cm away from the eye, which is 23 cm away from the lens because the lens is 2 cm in front of the lens, so do=23 cm. We want the image to be at the near point, which is 45 cm from the eye (43 cm from the lens), so di=43 cm. Using the lens equation above, that makes f=14.98 cm and using the equation for the power of the lens (with f in meters), the power is 6.673 D. This is incorrect, though, and I can't find the error in my process.
  2. jcsd
  3. Nov 10, 2011 #2
    I figured this one out; don't worry about it.
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