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Optics Question, where is the lens

  1. Jul 28, 2013 #1
    1. The problem statement, all variables and given/known data

    A 3 cm tall object is placed 11 m from a wall. If the image is to be projected upside down with a 20 cm focal lense, what is:

    A} The type of lens used?

    B] Where is the lens placed?

    C] What is the height of the image?

    D] What is the magnification of the lens?

    2. Relevant equations

    1/f = 1/d_i + 1/d_o

    3. The attempt at a solution

    I have gotten the answers:

    B] 0.2037
    C] -1.5899
    D] -69.999

    My textbook does not supply answers, therefore I am unsure if mine are correct.

    Thanks for the help,
  2. jcsd
  3. Jul 28, 2013 #2


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    Pls post your working.
  4. Jul 28, 2013 #3

    Simon Bridge

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    The way to check your working in optics is to sketch out the ray diagram and see if it appears to agree.
    You can also check be referencing related physics - i.e. how would you find the focal length of a lens using a very distant object? Therefore, how far should the lens be from the screen in your example (roughly)? Should it be farther or closer than that?

    - the lensmakers formula usually sits the lens at x=0, and other measurements relative to this. It is not clear where your results are measured from or how you have used the formula.
    - your results are meaningless without units.
  5. Jul 29, 2013 #4
    Thanks, I have added units.
    I sketched out a diagram, and ended up letting[itex] d_i = 11 - d_o [/itex]due to the problem restrictions.

  6. Jul 29, 2013 #5


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    And your working?
  7. Jul 29, 2013 #6

    Simon Bridge

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    ... and where you are taking these distances from?
    i.e. you wrote "B: 0.2037m" ... that does not actually answer the question does it?
    If you asked, "where is the toilet?" and I said "6m" - how would you react?

    The answer should look like: "The lens should be placed 20.4cm from ..." where? The screen? The object?

    ... and what about the other questions I asked?
    ... did you use your sketch to see if the answers you got made sense?
    i.e. that 20.4cm is very close to the focal length. Does that make sense?
    given that distance - does it make sense for the magnification to be so big?
    Last edited: Jul 29, 2013
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