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Lenses - magnification, position, focal length help

  1. Jul 9, 2012 #1
    Lenses - magnification, position, focal length help!!!

    ok so here is the quesiton..

    An object is loacted 3.5 cm from the optical centre of a lens. The lense produces an image of magnification that is +0.667 the size of the object

    a) is this lens a diverging or converging lens? explain

    b) Find the position of the image and the focal length of the lens using the magnification equation and the thin lens equation

    c) make a diagram of the situation, showing the axis of symmetry and the prinicple axis of the lens, its optical centre itrs principle focal points and the positions of the object and image



    ok so you said it was a diverging lens and i have NO IDEA how to do b.... i am going back and back to my notes... but i just dont understand :confused: ... someone please help me !!! this is due tomorrow too :cry:
     
    Last edited by a moderator: Jul 9, 2012
  2. jcsd
  3. Jul 9, 2012 #2

    berkeman

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    Staff: Mentor

    Re: Lenses - magnification, position, focal length help!!!

    Welcome to the PF.

    Per the PF rules, you need to show some effort before we can be of tutorial help.

    What do you mean "you said it was diverging"? Who is you?

    A good place to start is to list the lens equations, and show how you calculate magnification. Then try to make the sketch they are asking for -- that should get you pretty far along... Please show us your work.
     
  4. Jul 9, 2012 #3
    Re: Lenses - magnification, position, focal length help!!!

    So magnification is


    M= hi/ho = -di/do
     
  5. Jul 9, 2012 #4
    Re: Lenses - magnification, position, focal length help!!!

    So hi is +3.5cm. But what's the ho?

    M=3.5/? = +0.667

    So it would end up being 3.5/0.667 = 5.24? Am I correct?
     
  6. Jul 9, 2012 #5

    berkeman

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    Re: Lenses - magnification, position, focal length help!!!

    Good so far. Sketch?
     
  7. Jul 9, 2012 #6
    Re: Lenses - magnification, position, focal length help!!!

    Is there an equation for focal length.?
     
  8. Jul 9, 2012 #7
    Re: Lenses - magnification, position, focal length help!!!

    But I thought the magnification was +0.667?
     
  9. Jul 10, 2012 #8

    berkeman

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    Re: Lenses - magnification, position, focal length help!!!

    Yes, the lens equation.
     
  10. Jul 10, 2012 #9
    Re: Lenses - magnification, position, focal length help!!!

    Is it

    1/Di=1/f - 1/do
     
    Last edited: Jul 10, 2012
  11. Jul 10, 2012 #10
    But for that I am not sure where the numbers fit in. I do not have focal length. All I have is magnification and where the object is located
     
  12. Jul 10, 2012 #11

    I have 1/Di = 1/f - 1/3.5cm


    I am missing the focal length


    Wait that's the thin lens equation wow ok hold on.
     
  13. Jul 10, 2012 #12
    Re: Lenses - magnification, position, focal length help!!!

    No I need the thin lens equation .. I am so confused right now !!
     
  14. Jul 10, 2012 #13

    berkeman

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    Re: Lenses - magnification, position, focal length help!!!

    It would seem that you have two equations (lens equation and magnification equation) and two unknowns (a distance and the focul length)...

    http://en.wikipedia.org/wiki/Magnification

    .
     
  15. Jul 11, 2012 #14
    Re: Lenses - magnification, position, focal length help!!!

    Ok so I have my focal length as +4.3 magnification as 5.24 and distance of image is -18.3
     
  16. Jul 11, 2012 #15
    Re: Lenses - magnification, position, focal length help!!!

    How do I find position of image
     
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