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Convex lenses

  1. Mar 7, 2009 #1
    1. The problem statement, all variables and given/known data
    Given only the distance between the object and its image (p +q), how is it possible to find p, the distance of the object from the convex lens?

    2. Relevant equations
    1/p + 1/q = 1/f
    in this case, p + q =40
    also, the image is smaller than the original, so p > 2f

    3. The attempt at a solution
    I don't understand how to find p if q and f are unknown. I tried solving for q as q=40-p, but that didn't take me anywhere.
  2. jcsd
  3. Mar 7, 2009 #2


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    Homework Helper

    You need more information than you gave in part 1.
    How did you know the image is smaller than the original - do you know what fraction it is?
  4. Mar 7, 2009 #3
    It said if was 1/2 the size of the original. But how does that information help?
  5. Mar 7, 2009 #4
    Ahh, I think I get it. It has something to do with the similar triangles formed by the object/image and a light ray passing through the center of the convex lens, right?
  6. Mar 7, 2009 #5


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    Homework Helper

    Yes, or you can use the formula relating the object and image heights to their distances from the lens to get a 2nd equation relating p and q.
  7. Mar 7, 2009 #6
    Ok, thank you for your help!
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