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Homework Help: Geometric Optics - Converging Lens

  1. Oct 18, 2012 #1
    1. The problem statement, all variables and given/known data

    A certain lens focuses light from an object 2.90m away as an image 46.9cm on the other side of the lens.

    1. What type of lens is it? (Converging or Diverging?)
    2. What is the focal length?
    3. Is the image real or virtual?

    2. Relevant equations

    1/di + 1/do = 1/f

    3. The attempt at a solution

    1. I thought because the image formed on the other side of the lens the lens was diverging but my online homework assignment says otherwise (and so do lecture notes!)

    2. 1 / 0.029 cm + 1 / 46.9 cm = 1 / f

    3. Real (because it's a concave mirror, right?)
  2. jcsd
  3. Oct 18, 2012 #2


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    Science Advisor
    Homework Helper

    Hi PeachBanana! :smile:
    no, converging means that as time goes forward, the rays get closer

    light from an ordinary source always starts by diverging (it spreads out) … this lens focussed it :wink:
    i] it's a lens :rolleyes:

    ii] the image is real if you can put a screen there and see it on the screen

    that's why it's called "real"! :smile:
    a diverging lens makes the rays look as if they came from behind the mirror … but if you put a screen there, you'll see nothing, because the rays never went through there :wink:
  4. Oct 18, 2012 #3
    Thank you I understand much better.
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