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E&M question

  1. Apr 2, 2006 #1
    The distance between an object and its image is fixed at 40.0 cm. A converging lens of focal length f = 3.62 cm forms a sharp image for two positions of the lens. What is the distance a between these two positions?

    Determine:

    The distance between these two positions a = ____cm

    Could someone help me with the formula i want to be using?
     
  2. jcsd
  3. Apr 3, 2006 #2

    andrevdh

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    The thin lens formula gives
    [tex]\frac{1}{f}=\frac{1}{o}+\frac{1}{i}[/tex]
    with i the image distance and o the object distance. Now say you find a position between the object and fixed screen (image position) such that you get a sharp image on the screen. The object image is now o and the image distance i while the sum of these two being 40 cm. But according to the formula if you make the object distance i the image distance should be o! Which gives you the other position of the lens where you should get a sharp image on the screen. In the second case the object distance will therefore be i and the image distance will be o.
     
    Last edited: Apr 3, 2006
  4. Apr 3, 2006 #3

    andrevdh

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    I would guess that with a mech advantage of 2 you would have problems lifting your lecturer! Three would give you a fighting chance!
     
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