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Searching for a nice explanation: Plane Mirror

by DaTario
Tags: explanation, mirror, plane, searching
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DaTario
#1
Oct27-12, 04:53 PM
P: 555
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
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mathman
#2
Oct27-12, 05:31 PM
Sci Advisor
P: 6,112
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.
harrylin
#3
Oct27-12, 05:51 PM
P: 3,188
Quote Quote by mathman View Post
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.
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.

DaTario
#4
Oct28-12, 11:08 AM
P: 555
Searching for a nice explanation: Plane Mirror

Quote Quote by mathman View Post
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.
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.
DaTario
#5
Oct28-12, 11:17 AM
P: 555
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
mfb
#6
Oct28-12, 11:53 AM
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P: 12,111
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?
harrylin
#7
Oct28-12, 11:53 AM
P: 3,188
Quote Quote by DaTario View Post
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
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!
jtbell
#8
Oct28-12, 12:19 PM
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Quote Quote by DaTario View Post
Why a plane mirror image exchanges right with left but does not interfere with up and down?
See the attached picture. Are left and right exchanged between the plastic sheet and its reflected image?
Attached Thumbnails
LRTB.jpg  
harrylin
#9
Oct28-12, 12:58 PM
P: 3,188
Quote Quote by jtbell View Post
See the attached picture. Are left and right exchanged between the plastic sheet and its reflected image?
Nice one!
(Is that you?)
A.T.
#10
Oct28-12, 01:35 PM
P: 4,238
Quote Quote by mfb View Post
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.
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.

mathman
#11
Oct28-12, 03:21 PM
Sci Advisor
P: 6,112
Quote Quote by harrylin View Post
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.
Gravity has nothing to do with it.
mfb
#12
Oct28-12, 03:23 PM
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P: 12,111
Gravity influences the way our brain interprets images. See the video in the post above for an explanation.
A.T.
#13
Oct28-12, 05:02 PM
P: 4,238
Quote Quote by mathman View Post
Gravity has nothing to do with it.
That's true. Creatures living in zero-g and having a plane symmetrical body like humans would have the same confusion.
jtbell
#14
Oct28-12, 06:16 PM
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Quote Quote by harrylin View Post
Nice one!
(Is that you?)
Part of me, anyway.
DaTario
#15
Oct28-12, 07:34 PM
P: 555
Quote Quote by harrylin View Post
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!
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
DaTario
#16
Oct28-12, 07:42 PM
P: 555
Quote Quote by jtbell View Post
See the attached picture. Are left and right exchanged between the plastic sheet and its reflected image?
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
DaTario
#17
Oct28-12, 07:51 PM
P: 555
Quote Quote by A.T. View Post
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.

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
harrylin
#18
Oct29-12, 07:09 AM
P: 3,188
Quote Quote by DaTario View Post
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
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.


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