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Homework Help: Plano-concave thin lens, checking my result

  1. Apr 3, 2010 #1

    fluidistic

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    1. The problem statement, all variables and given/known data
    Determine the focal length of a plano-concave lens (n=1.5) which has a radius of curvature of 10 cm. What is its power in diopters?


    2. Relevant equations

    None given.

    3. The attempt at a solution
    1/f=(n-1)(1/R_1-1/R_2)=(1/2)(-0.1)=-1/20.
    Thus f=5 cm and P=-20 m^(-1).
    Is this right?
     
  2. jcsd
  3. Apr 3, 2010 #2

    Redbelly98

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    Hi,

    Your method is mostly correct, but you have made some arithmetic errors.
    Really? Because ...

    ... this looks like an equation to me :smile:
    Pretty much correct. R2 is considered negative, so -1/R2 would be +1/(20cm).
    (Edit: this is wrong, R2 is positive. See my next post.)
    Not quite. You went from 1/f = 1/20 (←correct) to f = 5 :confused:

    Try that last bit of algebra again, you are very close.

    RB
     
    Last edited: Apr 4, 2010
  4. Apr 3, 2010 #3

    fluidistic

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    I'm not given any tip about what equation to use, therefore I thought I might confuse someone willing to help me if I put a wrong equation in the "relevant equations". But from now on, I must use this section as an attempt to relevant equations. :smile:

    From my sketch R_1=infinity and R_2>0. Should I reach 1/f=-1/20?
    Ok for the rest, f=20 or -20 depending on my question.
    Any ray of light has to pass by the plane part and then the concave part according to my sketch. I'm sure I should reach 1/f=20, but I do not reach this. Should I recheck my calculations? I have (n-1)(-1/R_2) and as R_2>0, I get a negative result. Strange.
    Thanks for the help.
     
  5. Apr 4, 2010 #4

    Redbelly98

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    You're absolutely correct. For some reason, I read the question as saying "plano-convex". My mistake.

    It's -20, you were correct about it being negative. Of course, there should be units included when you report the answer.
     
  6. Apr 4, 2010 #5

    fluidistic

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    There's no problem. Thanks for the correction, your time and your help.
    As for the units, I think the ones in my first posts are OK.
     
  7. Apr 5, 2010 #6

    Redbelly98

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    Yes, your first post had correct units but wrong values for f and P.
     
  8. Apr 5, 2010 #7

    fluidistic

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    I get it, thanks. :smile:
     
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