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Question about optics and thin lenses

  1. Jan 28, 2010 #1
    1. The problem statement, all variables and given/known data
    Hey, I was just doing some questions on thin lenses, and thought of a question I really want answered just can't find anything on in the text or anywhere.

    So the question involves a lens with a focal length of f = 18.18 cm

    now the question asks if the lens would be able to produce an image on a screen that is 35 meters away. Now I can solve this by
    1/f - 1/q = 1/p
    stating that if you place the object 18.27 cm from the lens you can get an object at 35 meters that will be magnified and inverted, correct ?
    now my question is.. say we make the screen 300 meters away
    this equation shows that you will still get an image at 300 meters away by placeing the object 18.19 cm away from the lens? however the object will be hugely magnified, is this correct? im haveing trouble grasping this.. will the image still be in focus at that length ? it just doesnt seem practical.. even moving it to 500 meters i could still get an image.. when does it end? does light interference eventually wash out the image?

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Jan 28, 2010 #2


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    Science Advisor
    Homework Helper

    Yes, the equation tells you at what distance the image will be in focus.
    Or more usually in a camera how far to move the lens from the film to make an image of an object at a distance X

    Whether you could see the object of course depends on how bright it is, and how sensitive your eyes or the film are
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