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Homework Help: Converging / Diverging Lenses

  1. Jan 28, 2009 #1
    1. The problem statement, all variables and given/known data

    A converging lens has a focal length f, and a diverging
    lens has a focal length –f, which has the same magnitude
    as the converging lens. They are separated by a distance
    D which is greater than f, as shown. Parallel light enters
    from the left. Will the light be brought to a focus, and if
    so where?

    (basically there are two lenses. The converging lens is on the left of the diverging lens. The two are separated by some distance d. There are parallel horizontal rays traveling to the converging lens.)

    2. Relevant equations

    1/f = 1/do + 1/di ?

    3. The attempt at a solution

    I'm not really sure how to approach this question, but I would like to confirm some of my thoughts first (to avoid finding out I was wrong to think a certain way later). The parallel horizontal rays that hit the converging lens should go through the lens, pass through the focal point...and all be going from the focal point and out in a diverging manner.

    The light that's passed through the converging lens will then pass through the diverging lens, being refracted. This will form a virtual image in between the focal point and the diverging lens.

    However, the formation of this virtual image...is that considered as light being focused?

    Anyway...that's basically my thinking. Is this correct or am I missing something important?
    Last edited: Jan 28, 2009
  2. jcsd
  3. Jan 29, 2009 #2
    Your logic looks impeccable to me.
    The converging lens brings the light to a focus;
    The diverging lens doesn't.
    The question looks so easy, am I missing something?
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