Where is the principle focus of a plain mirror?

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Discussion Overview

The discussion revolves around the concept of the principle focus of a plain mirror, exploring whether such a focus exists and the implications of different types of mirrors on light behavior. The scope includes theoretical considerations and conceptual clarifications regarding optics.

Discussion Character

  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant asks about the location of the principle focus of a plain mirror.
  • Another participant describes the focal point as the location where parallel incident light converges, suggesting that if rays are slightly converging, they would focus between the mirror and the focal plane.
  • A third participant questions the validity of the term "plain mirror" and emphasizes the need to clarify whether such a mirror has a focal point before discussing its location.
  • One participant claims to have found that the focal point is at infinity, referencing a source.
  • Another participant points out that only parabolic mirrors focus light and that plane mirrors do not have a true focal point, suggesting that the concept of focusing at infinity is metaphorical.
  • A later reply speculates that plane mirrors can be considered as part of spheres with infinite radius, linking this to the idea of the focal point being at infinity.

Areas of Agreement / Disagreement

Participants express differing views on whether a plain mirror has a focal point, with some asserting it does not while others suggest it can be considered to focus at infinity. The discussion remains unresolved regarding the nature of the focal point in relation to plain mirrors.

Contextual Notes

There are ambiguities in the terminology used, particularly regarding "plain" versus "plane" mirrors, which may affect the clarity of the discussion. Additionally, the concept of focusing at infinity is presented as a metaphor rather than a definitive physical property.

chandubaba
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where is the principle focus of a plain mirror?
 
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chandubaba said:
where is the principle focus of a plain mirror?
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?

OR

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?
 
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...
 
I found the answer in resnick halliday website.its infinity
 
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.
 
Yes sir,plane mirror.I speculate that plain mirrors can be considered part of spheres with infinite radius and that's why its infinity.I suppose this the property of infinity.Sorry I forgot the URL of that site.pls search in a search engine.
 
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