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Ray-Tracing vs. Depending on Formulas

  1. Aug 11, 2014 #1
    A lens forms a real image of a lightbulb, but the image of the bulb on a viewing screen is blurry because the screen is slightly in front of the image plane. To focus the image, should you move the lens toward the bulb or away from the bulb.

    Formulas:
    1/s+1/s'=1/f

    Well, in the book, the answer is move away. It says we need to decrease s', so increase s.

    But I made drawings using ray-diagrams, and what I get is that, for an object further away from the focal point of a converging lens, moving the lens closer to the object would make more sense... The image plane actually gets closer to the object.

    There seems to be a contradiction here...
     
  2. jcsd
  3. Aug 13, 2014 #2
    I'm sorry you are not finding help at the moment. Is there any additional information you can share with us?
     
  4. Aug 14, 2014 #3

    ehild

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    According to the problem text, the object and the screen are at fixed positions, and the lens has to be moved to give sharp image. The distance between the object and image is d=s+s', and the distance between the screen and object is D. It is said that the screen is slightly in front of the image plane, which means D<d. You have to decrease d=s+s'. Derive the expression for d=s+s' and see if you need to increase or decrease s so as d decrease.

    ehild
     
  5. Aug 14, 2014 #4

    BvU

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    I mean to see a little snag here: usually there are two positions of the lens that yield a sharp image (provided d > 4f): s and s' are interchangeable.

    If the lens is closer to the object than to the screen, you can reduce s+s' by moving the lens away from the object.

    If the lens is closer to the screen than to the object, you can reduce s+s' by moving the lens away from the screen.

    Since we haven't been told which s is the distance between screen and lens and which is the distance between object and lens, nor whether the distance between screen and lens is smaller than the distance between object and lens, I end up considering the question ambiguous. Unless there's more information in the exercise than lingy mentions.
     
  6. Aug 14, 2014 #5

    ehild

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    @BvU: yes, it depends on the position of the object what we should do with the lens in order to get a sharp image. But I hoped that the OP would find it out.

    ehild
     
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