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Homework Help: Combination Lens Problem.

  1. Mar 20, 2013 #1
    1. The problem statement, all variables and given/known data

    An object is placed 400 mm in front of a convex lens of focal length 80 mm. Find the position of the image formed. State the nature of this image.

    A second concave lens of focal length 25 mm is placed 80 mm behind the first convex lens. Find the position of the final image formed and state its nature.

    3. The attempt at a solution

    [itex]\frac{1}{U} + \frac{1}{V} = \frac{1}{F} [/itex]

    [itex] \frac{1}{V} = \frac{1}{80} - \frac{1}{400} [/itex]

    [itex]V = 100 mm[/itex]

    So the Image from lens one forms 100 mm to the right of lens one. BUT, lens 2 is only 80 mm to the right of lens 1. I don't know what to do here since no image has been formed to create a new object for lens 2.

    Any help?
  2. jcsd
  3. Mar 20, 2013 #2
    Which means the Object´s image will appear to the right (or behind) of the concave lense.
    80-100= -20
    U=-20 solve for V
  4. Mar 20, 2013 #3
    [itex]\frac{1}{-20} + \frac{1}{V_{2}} = \frac{1}{F}[/itex]

    [itex]\frac{1}{V_{2}} = \frac{1}{20} - \frac{1}{25} = \frac{1}{100}[/itex]

    [itex] V = 100[/itex]
  5. Mar 20, 2013 #4
    Is this correct?
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