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Homework Help: Focal length

  1. May 1, 2010 #1
    The problem reads "What is the focal length (in meters) of a -4.2 diopter lens"

    The formula equation I have is = 1/f to get the diopter power

    -4.2 = 1/f and solve for f I get -.2381

    This is a homework problem where the solutions are entered on line. I entered the solution above and it didn't accept. Any hints/help?
  2. jcsd
  3. May 2, 2010 #2

    Doc Al

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    Staff: Mentor

    Looks right to me. But online systems are often fussy about things like significant figures: Try rounding it off to 2 sig figs. (Some systems insist on 3 sig figs.)
  4. May 2, 2010 #3
    Ok thanks. I know threwout the two semisters rounding has never been a factor but will try. Worst case if I use all my attempts I show teacher and get the 1 point for the problem. Thanks!
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