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Homework Help: 2 lenses

  1. Oct 5, 2008 #1
    1. The problem statement, all variables and given/known data
    A convex lens (f1=300mm) is placed 200 mm from a concave lens (f2=-50mm). An object is placed 6m away form the convex lens. (the order is as follows: object - 6m - convex lens - 200mm - concave lens).
    Determine the position, magnification and nature of a final image.

    2. Relevant equations
    for a single lens:
    1/x+1/y=1/f (where x is distance from the lens to the object, y is the distance from the lens to the image),

    3. The attempt at a solution
    Should I analyze 2 lenses separately?
    At first first lens, which gives the image at y1:
    y1=~31.5cm, so it is placed behind the second lens! Should I now consider the second lens, starting from the point, where is the image? I mean:
    where x2=20cm-31.5 cm=-11.5cm ?
    I'm a little bit confused..
    And finally, how to calculate the magnification?
  2. jcsd
  3. Oct 5, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Exactly right. Solve it in steps. The image from the first lens becomes the object for the second lens (virtual object, in this case, since it's behind the second lens).
    Find the magnification for each, then multiply them.
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