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Mirror equation help please

  1. May 17, 2009 #1
    Mirror equation help!! please urgent!!

    1. The problem statement, all variables and given/known data

    The distance from the lens of someone's eye to the retina is 20.0 mm. if the image of a book held 40.0 cm is in front of the eye in sharp focus, what is the effective focal length of the lense.
    the back of the book says the answer is 19 mm.

    thanks for your help everyone!

    2. Relevant equations
    1/f = 1/ do + 1/ di



    3. The attempt at a solution
     
  2. jcsd
  3. May 17, 2009 #2

    nrqed

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    Re: Mirror equation help!! please urgent!!

    Just plug in the values.

    Don't forget that 40 cm = 400 mm
     
  4. May 17, 2009 #3
    Re: Mirror equation help!! please urgent!!

    Gahh! that's why it didn't work.

    I forgot to convert Units.

    Ahhh i'm a silly physics 11 student.

    Thanks for your help!

    Hmm does the distance from the retina to the lense (20 mm ) count as the Do ?
     
  5. May 17, 2009 #4

    nrqed

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    Re: Mirror equation help!! please urgent!!

    You are welcome:smile:

    No, 20 mm is Di, the distance to the image (which is on the retina). 400 mm is the object distance
     
  6. May 17, 2009 #5
    Re: Mirror equation help!! please urgent!!

    Thanks!

    So glad I join this message board.

    My physics 11 teacher is horrendous, our class average is in the 50's.

    I'm managing a 'B', but it's been difficult.
     
  7. May 17, 2009 #6

    nrqed

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    Re: Mirror equation help!! please urgent!!

    You are welcome. Sorry to hear about your situation! I wish you the best luck!
     
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