cjh@petsd.UUCP (Chris Henrich) (01/22/88)
In article <760@aucs.UUCP> 820785gm@aucs.UUCP writes: > > So just why DOES a mirror reverse left and right, but not up and down? Here's an explanation I find satisfying; I read it in some fairly level-headed magazine. The mirror, by itself, reverses front-to-back. Our sense of vision, seeing the result, mentally applies a rotation to the mirror image. (Why? I suppose, to get it as close as possible to one's "body image.") The axis chosen is from head to feet, in most circumstances. The combination of the front-to-back reflection and the vertical rotation is a left-to-right reflection. Regards, Chris -- Full-Name: Christopher J. Henrich UUCP: ...!hjuxa!petsd!cjh US Mail: MS 313; Concurrent Computer Corporation; 106 Apple St; Tinton Falls, NJ 07724 Phone: (201) 758-7288 Concurrent Computer Corporation is a Perkin-Elmer company. "Life is what the least of us make most of us feel the least of us make the most of." -- Quine
rob@amadeus.UUCP (01/24/88)
C. J. Henrich writes: >In article <760@aucs.UUCP> 820785gm@aucs.UUCP writes: >> >> So just why DOES a mirror reverse left and right, but not up and down? >Here's an explanation I find satisfying; I read it in some fairly >level-headed magazine. > >The mirror, by itself, reverses front-to-back. Our sense of vision, >seeing the result, mentally applies a rotation to the mirror image. >(Why? I suppose, to get it as close as possible to one's "body >image.") The axis chosen is from head to feet, in most >circumstances. Not quite true. Mirrors do reverse front-to-back. But we interpret our body image as if we were meeting someone else. Thus we expect our image's right hand to be on the left. Since it's on the right (where it should be but not expected) we think that it's reversed left-to-right. --- Dan Tilque This is a borrowed account, so be sure to indicate that replies are for me and not for Rob.
herb@mit-caf.UUCP (Herbert Neuhaus) (01/24/88)
In article <3033@zeus.TEK.COM> rob@amadeus.UUCP (Dan Tilque) writes: >C. J. Henrich writes: >>In article <760@aucs.UUCP> 820785gm@aucs.UUCP writes: >>> So just why DOES a mirror reverse left and right, but not up and down? >>The mirror, by itself, reverses front-to-back. Our sense of vision, >>seeing the result, mentally applies a rotation to the mirror image. >>... close as possible to one's "body image. ... >Not quite true. Mirrors do reverse front-to-back. But we interpret our >body image as if we were meeting someone else. Thus we expect our image's >right hand to be on the left. ... If our interpretation of mirror images is related to expectations of meeting "someone", then why do inanimate objects seem to beahve the same way in mirrors as people do? I agree the trick is in our sense of vision, but reject the idea that its an anthropomorphic reaction. Consider the image of a object which is lying on its side. Is the image reversed from top to bottom or left to right. If you say its left to right, would that be with respect to you (upright) or the with respect to the object which is on its side. The answer is that text is always backwards in the mirror, no matter how you rotate it. Perhaps somethings are not symmetric (people, text) to mirror inversion, and other things are (like gravity?). What do you think? Herb ( on his side in front of a mirror).
ir353@sdcc6.ucsd.EDU (Matthew Grayson) (01/25/88)
In article <676@mit-caf.UUCP> herb@mit-caf.UUCP (Herbert Neuhaus) writes: >In article <3033@zeus.TEK.COM> rob@amadeus.UUCP (Dan Tilque) writes: >>C. J. Henrich writes: >>>In article <760@aucs.UUCP> 820785gm@aucs.UUCP writes: >>>> So just why DOES a mirror reverse left and right, but not up and down? >>>The mirror, by itself, reverses front-to-back. Our sense of vision, >>>seeing the result, mentally applies a rotation to the mirror image. >>>... close as possible to one's "body image. ... >>Not quite true. Mirrors do reverse front-to-back. But we interpret our >>body image as if we were meeting someone else. Thus we expect our image's >>right hand to be on the left. ... > >If our interpretation of mirror images is related to expectations of meeting >"someone", then why do inanimate objects seem to beahve the same way in >mirrors as people do? > >I agree the trick is in our sense of vision, but reject the idea that >its an anthropomorphic reaction. >..... >What do you think? > >Herb ( on his side in front of a mirror). Suppose that you had a book in which the pages are printed with random orientations, some reversed, some rotated. You open the book at random and aim it at the mirror. You see a page in the mirror, and you ask yourself: What does it look like on the page? Slow Motion now.... You turn the book around! Any rotation in an odd dimensional space has a fixed axis. In order to see the real page, you must choose an axis about which to rotate. O.K., you say, I'll leave the book still and go look.... No good, you still have to turn around somehow, and that turning requires a choice of orientation. It's impossible to see the real page without making this choice. If you turn the book around a vertical axis, it appears reversed left-right. If you turn it around a horizontal axis, it's upside-down. Axes at other angles yield different answers. Ahah! I'll make the book transparent and look straight through it! Surprise! I see exactly the same image as in the mirror! This yields a disturbing conclusion. Mirrors don't reverse images at all, they reverse orientation (we see the back of something instead of the front). In order to convert the mirror image to a front view, we introduce the choice of axis which results in a reversed (or upside-down or whatever) image. Matt
pax@uiucdcsp.cs.uiuc.edu (01/25/88)
A mirror doesn't reverse left to right nor up to down. The fact that inanimate objects like text appear reversed, is that you turned the book around yourself to hold it up to the mirror. If the book were transparent and you tried to read it while holding it up to the mirror, you would be able to see that the letters are reproduced in exactly the way you see them through the transparent book. Here is an interesting fact about mirrors: The amount of your body you can see in a mirror does not vary with your distance from the mirror, if you stand back you can't see a greater proportion of yourself.
al@gtx.com (0732) (01/27/88)
(The question was why a mirror appears to reverse left and right,
not up and down)
If we apply a certain type of linear transformation (a reflection) to
certain asymmetrical objects, the result may not be congruent to the
original object. A reflected glove, for example, is not congruent to
the original glove. These objects are said to possess "handedness",
and the handedness is reversed by the reflection.
We use the words "left" and "right" as handedness-dependent directions
relative to certain asymmetrical objects. The position of the steering
wheel of a car, for example, defines directions called "left" and
"right" relative to the car.
Now, just as a mirror does not "reverse up and down", it does not
"reverse left and right" as directions. If I look into a mirror and
point up, my image points up. If I look into a mirror and point to my
right, my image points to my right.
The confusion arises because my image has opposite handedness from me,
and although we agree on what is "up", my "right" is different from its
"right" because we have different handedness.
To sum up, a mirror appears to reverse right and left because the words
"right" and "left" are defined relative to the handedness of a real or
virtual object, and my reflection has opposite handedness.
----------------------------------------------------------------------
| Alan Filipski, GTX Corp, 2501 W. Dunlap, Phoenix, Arizona 85021, USA |
| {ihnp4,cbosgd,decvax,hplabs,amdahl}!sun!sunburn!gtx!al (602)870-1696 |
----------------------------------------------------------------------smythe@batcomputer.tn.cornell.edu (Bradford Smith) (01/28/88)
As has been observed, a mirror reverse FRONT/BACK, not left right as it appears. The reason for the confustion is that in everyday life, left and right are defined in terms of front and back and top and bottom are not. Therefore, reversing front/back will reverse left/right but not top/bottom. The reason text is always reverse is because in order to read it, you must look at the "front" of the letters, not the back, which is what is seen when text is seen in a mirror. The way I visualise mirror reversal is to picture seeing the FRONT of the object as if I were looking at it from the back. This is what a mirror does. It is difficult to visualise (even more difficult to describe) because one expects the rest of the three dimensional body to be in the way. It is not in a mirrir because a mirror is 2-D. So, to understand why left/right are reversed, all that need be done is to picture the object as two dimensional and look at it from the other side. No probelm. :-) -Bradford
NU109703@NDSUVM1.BITNET (01/28/88)
The reason I think a mirror reverses left to right instead of up and down:
The mirror itself has no 'preferences' as to which way to reverse. It is
only a simple reflection of light. The reason that we may "say" it reverses
us is because we see the difference between the way we see ourselves and the
way others see ourselves. If people normally stood on their heads to look at
each other (I know it couldn't make sense with more than 2 people), then the
mirror would be seen to reverse your image up and down. You would then be
asking, "Why does a mirror reverse your image up and down and not left and
right?" The only difference, in my point of view, is the difference in
reference (of your image) between you and all other people.ir353@sdcc6.ucsd.EDU (Matthew Grayson) (01/28/88)
In article <3053@zeus.TEK.COM> rob@amadeus.UUCP (Dan Tilque) writes: >Matthew Grayson writes: >>........ Mirrors don't reverse images at all, >>they reverse orientation (we see the back of something instead of the front). >........ >An interesting point of view [sorry, I couldn't resist the pun]. Anyway, >mirrors *do* reverse front-to-back. > >Try holding the page at right angles to the mirror. Now the mirror *is* >reversing the writing. >--- >Dan Tilque Sorry if I wasn't being clear. We are saying the same thing. Orientation reversal does not lead to reversed images until an axis of rotation is chosen. Mirrors reverse front to back (that is, after all, the component of direction being reversed in the light path). You and I and everyone else who has posted an answer have stated as much (see my excerpt above, and your reply.) Now let's move on to the question of why telephone cords get those travelling helicity reversals :-) Matt
cosell@cosell.bbn.com (Bernie Cosell) (01/28/88)
In article <5127@spool.cs.wisc.edu> matt@cs.wisc.edu (Mad Matt Schaefer) writes: >The point that seems to have been missed (but I'm not sure, since I may not >have read all of these mirror articles too closely) is that because our eyes >are situated on a horizontal line relative to our head, front/back reversal will >cause left/right reversal. If our eyes were vertically aligned, let's say one on >the forehead and one on the nose, then front/back reversal would cause top/ >bottom reversal. Or something like that. Does that mean that if you close one eye there'll be no reversal at all? :-) __ / ) Bernie Cosell /--< _ __ __ o _ BBN Labs, Cambridge, MA 02238 /___/_(<_/ (_/) )_(_(<_ cosell@bbn.com
rwood@polyslo.UUCP (Robin Wood) (01/29/88)
ok. let's try to finish this discussion and get back to 'humor'
(I do remember what that is, I think). The real reason you perceive
yourself 'backwards' in the mirror is because little green men pick up your
image and turn it around really fast just to confuse you. It is quite
similar to the refrigerator light effect.
and if you don't believe that, there is also another common theory that
has to do with the fact that most of us have two eyes. If our eyes were placed
one above the other, then the little green men would make us look upside down
in the mirror, not backwards. REALLY.
A mind is a terrible thing to waste on,
--
o< - robin
\_/ matt@shorty.CS.WISC.EDU (Mad Matt Schaefer) (01/29/88)
In article <20215@bbn.COM> cosell@bbn.com (Bernie Cosell) writes: >In article <5127@spool.cs.wisc.edu> matt@cs.wisc.edu (Mad Matt Schaefer) writes >>[ ] >>the forehead and one on the nose, then front/back reversal would cause top/ >>bottom reversal. Or something like that. > >Does that mean that if you close one eye there'll be no reversal at all? :-) As I went to sleep the night after I posted the article about the eyes, I thought the same thing and decided that what I had posted was utter BS. I guess that will teach me to post without sleep. Anyway, I say there is no left-right reversal with respect to the person looking in the mirror, since as was pointed out earlier, if you point to your left, your image points to *your* left as well. However, if we compare the images, we get both horizontal *and* vertical reversal. Let's say you take a camera and take a picture of yourself looking in a mirror. Let's say, for the sake of demonstration, that your image's camera takes a picture of you at the same time. After the film is developed, we take the two pictures and place them side by side with both heads on top. There is left-right reversal, as if there was only one picture with a mirror perpendicular to it at its side. Then take the two pictures and put them head-to-head. There is top-bottom reversal, as if the perpendicular mirror was on the top of one picture. So depending on your point of view (I wonder what your image sees ;^) there is either no reversal or total reversal. Matt Schaefer ...!{harvard,ihnp4,rutgers,ucbvax}!uwvax!matt UW-Madison Computer Sciences Laboratory matt@cs.wisc.edu
throopw@xyzzy.UUCP (Wayne A. Throop) (01/30/88)
> matt@shorty.CS.WISC.EDU (Mad Matt Schaefer) > The point that seems to have been missed (but I'm not sure, since I may > not have read all of these mirror articles too closely) is that because > our eyes are situated on a horizontal line relative to our head, > front/back reversal will cause left/right reversal. If our eyes were > vertically aligned, let's say one on the forehead and one on the nose, > then front/back reversal would cause top/ bottom reversal. Or something > like that. This is a remarkably poor explanation. Consider a person with one eye. Would such a person be equally likely to flip left-right and up-down in a mirror? I doubt it, because the left-right flip of a human body is "symmetrical", while the up-down flip is not. Is somebody who has two eyes in a vertical line above the nose apt to flip so feet become head rather than hand becomes hand? Implausible indeed. Much more likely is the "you treat your image as if you were meeting another person" explanation. Your image then seems rotated 180 degrees about the vertical axis and reflected left-to-right. Which, if you go through the transforms, is identical to being reflected front-to-back. -- "You think this is a trap, then?" the Count asked. "I think everything is a trap until proven otherwise," the Prince answered. "Which is why I'm still alive." --- from "The Princess Bride" by William Goldman -- Wayne Throop <the-known-world>!mcnc!rti!xyzzy!throopw
venky@pitt.UUCP (S. Venkatesan) (02/04/88)
In article <73600006@uiucdcsp> pax@uiucdcsp.cs.uiuc.edu writes: > > >Here is an interesting fact about mirrors: The amount of your body >mirror, if you stand back you can't see a greater proportion of yourself. Another interesting fact: To see yourself fully, the height of the mirror should be at least half your height. Of course, higher than that will not help because of the above fact.