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How to determine a lens width?

  1. Dec 18, 2009 #1
    I am wondering... say an object was put directly on a lens and you wanted the projected image to focus at around 3 cm on the other side. How do you determine how wide the lens has to be?
    (please see attachment)
     

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  2. jcsd
  3. Dec 18, 2009 #2
    "how wide" in itself is not enough to specify this, since the angle of incedence depends on the curvature of the lens. you must look for a lens wth the correct focal length, where
    1/f = 1/u + 1/v (if i'm remembering correctly)

    the top bit of this page explains...
    http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/lenseq.html
     
  4. Dec 18, 2009 #3
    Ah, so it is also determined by the shape. However, i expect it would need to be a convergent lens type and assume it needs to be like a () shaped lens. The problem is however, that this equation can help find focal length but not describe how the lens itself will be shaped.
     
  5. Dec 19, 2009 #4

    Andy Resnick

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    Placing an object directly in contact with a lens (any lens) is not a good way to form an image any finite distance away from the lens. Why do you want to do this?
     
  6. Dec 19, 2009 #5
    Simply curious if its possible. I'm not sure it is, which is why i'm wondering. What about if the object is a very small distance from the lens, like say 1 mm. Can you focus the image on the other side 3 cm away?
     
  7. Dec 19, 2009 #6

    Andy Resnick

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    Sure, if the focal length of the lens is correct. That's how a microscope objective works.
     
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