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Where is the principle focus of a plain mirror?

  1. Oct 27, 2006 #1
    where is the principle focus of a plain mirror?
  2. jcsd
  3. Oct 27, 2006 #2


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    The principle focus or focal point of a mirror is the point at which light that is incident parallel to the aixs comes together at a point. The focal point is in a plane perpendicular to the axis (parallel to the surface of a plane mirror) called the focal plane. Any parallel rays incident on the mirror surface come to focus in the focal plane. If the incident rays are even the slightest bit converging when they hit the mirror, they will come to focus between the focal plane and the mirror.

    What happens to parallel rays when they hit a plane mirror? Where would they come to focus if they were slightly converging? If you start with converging rays, and then imagine making them less and less convergent, what happens to the point where they come into focus?


    Suppose you have a mirror that is convergent, so parallel incoming rays come to a focus in the focal plane. Imagine making the mirror flatter (less convergent). What happens to the focal point?
  4. Oct 27, 2006 #3
    Maybe you should ask:

    1) Is there such a thing as a plain mirror
    2) Does a plain mirror have a focal point
    3) where is that focal point

    It is meaningless to ask the third question without ascertaining the answer of the first two.
    If you decide to ask Q3 you must limit your question to a hypothetical situation and remove some ambiguities in my opinion.

    I think that's what Dan was getting at too...
  5. Dec 1, 2006 #4
    I found the answer in resnick halliday website.its infinity
  6. Dec 1, 2006 #5


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    Do you possibly mean a plane mirror? (I would think that most "plain" mirrors are "plane" mirrors!) Only parabolic mirrors focus light. Mirrors that are not parabolic do not focus light and so do not have a "focal point".
    You can, if you wish, think of a plane mirror as focusing "at infinity" but please remember that that is only a figure of speech.
  7. Dec 1, 2006 #6
    Yes sir,plane mirror.I speculate that plain mirrors can be considered part of spheres with infinite radius and thats why its infinity.I suppose this the property of infinity.Sorry I forgot the URL of that site.pls search in a search engine.
    Last edited: Dec 1, 2006
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