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Image from object at a focal point of mirror

  1. Feb 6, 2014 #1
    Hi all,
    If I place an object far away from a converging mirror or lens and start to move it closer, I note that I get an inverted, real image that steadily grows in size. As the object crosses the focal point the image becomes incredibly large and blurry and then flips upright and begins to shrink in size.

    My question concerns what's going on with the image of the object right when the object is at the mirror's focal point. I understand, based on tracing rays, that diverging rays eminating from a point on the object in this location will interact with the mirror so that they reflect as parallel rays and thus will never converge to an image point either in front nor behind the mirror.

    My confusion is that I am able to focus on extremely distant objects like stars (often said to be "at infinity")and form an image in the back of my retina of these objects. These objects are so distant that their rays are effectively parallel.

    So what is different about the reflected light from an object placed at the focal point of a mirror that prevents my eye from being able to focus this light to form an image on my retina? I sense I'm missing something fundamental here.


    Last edited: Feb 6, 2014
  2. jcsd
  3. Feb 6, 2014 #2


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    Staff Emeritus
    Science Advisor

    You will easily be able to focus on an object placed at the focal point of a mirror/lens. Remember that your eye adds another set of optical elements to the mix so it's not simply a single lens/mirror situation anymore.
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