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Magnification (Lens) Physics

  1. Jul 25, 2011 #1
    1. The problem statement, all variables and given/known data

    A convex lens and a concave lens both of 6cm focal length are placed to give an object which is 5cm from the convex lens of magnification of 10. How far are the lenses apart?

    2. Relevant equations

    1/f = 1/u + 1/v

    3. The attempt at a solution

    Lens One: Convex


    1/f = 1/u + 1/v
    1/6 = 1/5 + 1/v

    Lens Two: Concave

    f=-6 (concave)
    To get a magnification of 10:
    10 = v/u

    That's all I have. I have no idea how to work out the distance between the two lenses. Can anyone help please?
  2. jcsd
  3. Jul 25, 2011 #2


    User Avatar
    Homework Helper

    You also have
    magnification = V/U = 30/5 = 6. So the image of the first, convex lens is 6 times as large as the object.

    It really helps to sketch the lenses and draw 2 or 3 rays to determine the location and size of the image. You will see what is going on if you do this.

    The image of the first lens is the object for the second.
    1/f = 1/u + 1/v magnification = v/u
    You know f and can use the second equation to get v as a number times u. You should be able to solve these two to get a number for u. With that, you can add the second lens to your sketch and figure out the distance between the two lenses. Good luck!
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