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B Infinity Focal Points of Plane Mirror

  1. Apr 3, 2016 #1
    Hello!

    I've read on several pages that plane mirrors have an infinite amount of focal points. I don't understand? I thought plane mirrors have no focal points because the rays are parallel and don't focus in the first place. Why does a plane mirror have infinity focal points and what does it mean?
     
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  3. Apr 3, 2016 #2

    blue_leaf77

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    Can you provide the links to those pages?
    Without further information about the context being discussed, I can only interpret your issue as you missunderstanding the focal plane of a plane mirror being located at infinity as a plane mirror having infinite number of focal points.
     
  4. Apr 3, 2016 #3
    upload_2016-4-4_8-13-19.jpeg upload_2016-4-4_8-14-47.jpeg upload_2016-4-4_8-16-23.jpeg
    Yeah, I'm just struggling a little bit ot understand what it means to have an infinate amount of focal points. I do know that concave mirrors and convex glasses have focal points where all the rays meet. If the light rays hit the concave mirror parallel to the axis they will all pass through the focal point, and the rarefaction of the convex glass will direct the rays to a focused point. But a plane mirror? How is that possible? Is it because the rays can hit the glass at any angle and create multiple focal points because of which way two or more rays approach the plane mirror?
     
  5. Apr 3, 2016 #4

    blue_leaf77

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    Isn't it clear from those pages that you were just misunderstanding the term? None of them tell you that a plane mirror has an infinite amount of focal points.
     
  6. Apr 4, 2016 #5
    Oh sorry.... I must have mixed up the words. I was probably meaning focal length, not focal points. My bad, terribly sorry. I thought a plane mirror has an infinite focal length because it can display an image from as far as the image is while concave/convex objects can only display perfect images (at least clear) when their rays meet at a point. I don't know if that's correct or not. And the last sentence of the Wikipedia text says that the focal length (sorry I meant length) is infinite.
     
  7. Apr 4, 2016 #6

    blue_leaf77

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    Yes, the focal length of a plane mirror is at infinity. It comes from the extension of the focal length of a spherical mirror of radius of curvature ##R##, which is ##f=R/2##. A plane mirror has ##R \to \infty##, so ##f \to \infty## too.
     
  8. Apr 9, 2016 #7
    Ohhh ok. So because R is infinite, then the focal length must be infinate too. And this is for a plane mirror. I think I get it now. Thank you for your help!!! I really appreciate it. :smile:
     
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