Searching for a nice explanation: Plane Mirror

  1. Hi All,

    The question is rather simple: Why a plane mirror image exchanges right with left but does not interfere with up and down?

    Best wishes,

    DaTario
     
  2. jcsd
  3. mathman

    mathman 6,656
    Science Advisor
    Gold Member

    It is simply a definition question. Suppose you look at a mirror while lying down on your side. Then the exchange will be top and bottom and not right and left.
     
  4. Right. A little more elaboration: it is a very strong suggestion due to a combination of two factors:

    1. the (almost) left-right symmetry of faces
    2. gravity

    As a result your face in the mirror looks like a person who has turned around a vertical axis by 180°, as we are used to. If you do the same with for example playing cards then you can turn it also around the horizontal axis and the illusion is gone. :smile:
     
  5. Still the image of this person will experience an exchange between his left and right. The corresponding experiment in which the person remains still and the mirror rotates 90 degrees shows no apparent modification as well.
     
    Last edited: Oct 28, 2012
  6. Imagine a face of being as a compact disc with four marcs of different colors located at the edge: blue for (our) up, red for (our) down, green for (our) left and yellow for (our) right. Let his eyes be unique and located at the center of the disc. Once he is right in front of some plane mirror what should be his testimony. Will he declare that the image has only exchanged green and yellow?

    Best wishes

    DaTario
     
  7. mfb

    Staff: Mentor

    A mirror exchanges "front" and "back". If you rotate a human by 180° along its vertical (head<->feet) axis and exchange "front" and "back" afterwards, it is similar to an exchange left<->right. But that is an arbitrary way to handle directions, to make the feet pointing "down" again.

    If something in the mirror appears to be above/below your head, the object will be above/below your head.
    If something in the mirror appears to be left/right of your head, the object will be left/right of your head.
    Do you see the symmetry?
     
  8. Sorry that doesn't make sense to me in view of my explanation in post #3 (to which you did not respond). For symmetry you would need to put 4 eyes; else it is you who makes the effect. Or you could try with drawing one eye above, and one eye beneath!
     
  9. jtbell

    Staff: Mentor

    See the attached picture. Are left and right exchanged between the plastic sheet and its reflected image?
     

    Attached Files:

  10. Nice one! :biggrin:
    (Is that you?)
     
  11. A.T.

    A.T. 6,281
    Science Advisor
    Gold Member

    This is the key, to the confusion. The brain is used to assume rotations, not mirroring. So given certain symmetries it wrongly assumes that the mirror image is rotated by 180° along a certain axis, rather than just mirrored at the plane. This assumed rotation makes it then difficult to interpret what was exchanged by the mirror.

    https://www.youtube.com/watch?v=msN87y-iEx0
     
    Last edited: Oct 28, 2012
  12. mathman

    mathman 6,656
    Science Advisor
    Gold Member

    Gravity has nothing to do with it.
     
  13. mfb

    Staff: Mentor

    Gravity influences the way our brain interprets images. See the video in the post above for an explanation.
     
  14. A.T.

    A.T. 6,281
    Science Advisor
    Gold Member

    That's true. Creatures living in zero-g and having a plane symmetrical body like humans would have the same confusion.
     
    Last edited: Oct 28, 2012
  15. jtbell

    Staff: Mentor

    Part of me, anyway. :wink:
     
  16. Sorry for not responding. I was trying to focus on contributions upon which I could elaborate more. But thank you a lot. I believe that the number of eyes has no determinant role to play in this discussion.

    Best wishes,

    DaTario
     
  17. If the L letter you draw was a man, he will understand its image as facing him with the left-right exchanged. But now I guess we here touched a fundamental point. Mirrors provide front-back exchange. This effect can be presumed by the simmetry the plane have. And this front-back interplay yields this interpretation of left-right exhange.

    In terms of a nice explanation, I guess we are pretty near.

    Best wishes,

    DaTario
     
  18. Very nice. I my self recognized this video as the source of my question here. I was not remembering from where I took this question. For a moment I thought I have reached this problem independently, but I watched this video a couple of years ago and now I guess the problem is solved to me. Thank you.

    Best Regards,

    DaTario
     
  19. I also believe that the number of eyes has no determinant role to play in this discussion. I explained that your face in the mirror looks like a person who has turned around a vertical axis by 180°, as we are used to. If you do the same with for example playing cards then you can turn it also around the horizontal axis and the illusion is gone. However, it seems that somehow you suddenly understood it. :wink:
     
  20. jtbell

    Staff: Mentor

    To see more clearly where the usual left-right reversal in a mirror image comes from, first note that in my picture, the letters are written on the side of the plastic sheet that faces me. Now imagine replacing the clear plastic with cardboard. We no longer see the letters in the mirror because they're on the "wrong" side of the sheet, facing away from the mirror.

    To make the letters visible in the mirror, we need to flip the sheet around. If we flip the sheet horizontally (around a vertical axis), we see the usual "mirror-reversed" image. What happens if instead, we flip the sheet vertically (around a horizontal axis)? :smile:
     
  21. A.T.

    A.T. 6,281
    Science Advisor
    Gold Member

    I think that even with a completely asymmetrical object, the brain still tries to interpret it as rotation first. It's just what it does all the time when your eyes are open, and you recognize familiar objects from different angles. Recognizing mirrored objects is used less.
     
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