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Homework Help: Problem regarding lenses

  1. Apr 27, 2014 #1
    1. The problem statement, all variables and given/known data
    Which of the following is a characteristic of a farsighted person (nearby objects appear blurry)?
    a)the minimum focal length they can get for the lens of their eye is not small enough.
    b) the maximum focal length they can get for the lens of their eye is not large enough
    c) their lens of their eye is not symmetrical

    2. Relevant equations
    1/f = 1/p + 1/q

    f = 1/(1/p + 1/q)

    p = 1/(1/f-1/q)

    f is focal length, p is object distance, q is image distance
    3. The attempt at a solution

    The answer is B. I agree, but I don't understand my professor's explanation.

    The explanation for B being correct:
    "with q fixed, then to make p small f --> large. If can't get f --> large then can't get p --> small."

    My reasoning - I found that when P increases, F must increase, which is the opposite of the explanation. If when P decreases F decreases, and if when P is small the objects are blurry, F must be too small.

    If someone can confirm or deny the validity of my reasoning and explain the explanation given by my professor it would be much appreciated.
  2. jcsd
  3. Apr 27, 2014 #2
    The correct answer is A.

    Your professor made a mistake, maybe?
  4. Apr 27, 2014 #3
    He certainly may be wrong, though it is unlikely. Where is the flaw in my reasoning?
  5. Apr 27, 2014 #4
    You said it yourself. You got the opposite effect as the one used in the explanation. Small p requires small f. If it's blurry it's because they couldn't produce such a small f because "the minimum focal length they can get for the lens of their eye is not small enough". Notice how I concluded the logical statement with the quoted answer from item A. A is the correct answer.
  6. Apr 27, 2014 #5
    Ah, you are right. I confused myself. It's clear now. Thanks!
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