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An object placed in front of a plane mirror

  1. Nov 18, 2008 #1
    An object is placed 1m in front of a plane mirror. An observer stands 3m behind the object. For what distance must the observer focus his eyes in order to see the image of the object?

    My guess is that the observer would have to focus their eyes 3m in order to see the image because when the object is reflected off the mirror it is reflected at the same angle so therefore it reflects back at that same 1m spot. I am not quite sure if this is right though.

    Any help would be appreciated!
  2. jcsd
  3. Nov 18, 2008 #2


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    He would need to focus at 3m to see the object itself, but not it's image. The image does not focus at the same point as the object. (If this was the case we wouldn't see anything in the mirror!)

    The image of the object is actually a virtual image that appears to be behind the mirror. How far behind the mirror does the image seems to be?

    Add this to the distance the observer is from the mirror, and you have your answer.
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