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Easy way to remember imaging properties of lens?

  1. Oct 21, 2005 #1
    Is there an easy way to remember what kind of images will be formed from converging/diverging lens depending on where you place the object relative to the focus? For instance, how to tell if the image is inverted vs. erect, real vs. virtual, smaller vs. magnified? I always have a hard time remember what happens in the different cases. I know that a diverging lens always forms a virtual, erect image that is smaller than the object.
    Last edited: Oct 21, 2005
  2. jcsd
  3. Oct 22, 2005 #2

    Chi Meson

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    Diverging lenses are easy: all images are virtual, upright, and reduced.

    With converging lenses, it depends on the object distance, but there are five possible outcomes:

    With the object between the lens and the focal length (F), the image is virtual, upright, enlarged.

    With the object right at F, there is no image.

    With the object between F and twice the focal length (2F), the image is real, inverted, enlarged.

    With the object at 2F, the image is real, inverted, and same size as object.

    With the object beyond 2F, the image is real, inverted, and reduced.

    Notice that "real, inverted" and "virtual, upright" always go together.

    Here's a great ray-tracing aplet to play with:
    http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=65 [Broken]
    Last edited by a moderator: May 2, 2017
  4. Oct 22, 2005 #3
    Understand what happends to the rays as your object moves away and toward s the lens. This way you don't have to rely purely on memory. When you work more with the material it will come quicker. This way you can also check your answers and know if they're in the right "ballpark."

    This applet will help: http://www.rawbw.com/~xmwang/physDemo.html#java2d [Broken]
    Last edited by a moderator: May 2, 2017
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