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Homework Help: Grade 11 optics

  1. Dec 17, 2008 #1
    A candle is placed 1.8 m from a screen. whereshould a lens of focus length 21 cm (0.21 m) be placed between the screen and the candle to prduce a sharp image screen?

    The only formula i could think of was 1/f=1/do+1/di

    I've now spent 3 hours on this (test tomorrow) and still don't know where to start. I have that first formula and the only other formula we've used had to do with magnification and object height. I am truly lost.
  2. jcsd
  3. Dec 17, 2008 #2


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    Homework Helper

    Welcome to PF.

    Looks like the thin lens formula applies here doesn't it?

    So ... won't the distance x be able to be found by

    1/x + 1/(1.8 - x) = 1/.21 ?
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