Order of Kilopi
This has appeared in books, but I'm interested in what new insights we can come up with: When you write words on a card and hold them to a mirror, why does the mirror reverse left to right and not top to bottom? Is there any connection to why our eyes are side by side instead of one over the other?
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The mirror is not really reversing the image per se. This is not exactly how mirror work but the analogy should help you understand what you are seeing. Imagine the piece of paper is emitting light, and you are standing behind the piece of paper. If the light flies straight ahead and touches a wall, what you will get is an image of the text where whatever is on the left hand side of the paper relative to you will remain on the left-hand side of the image on the wall. Everything on the right-hand side of the image relative to the you will be on the right-hand side of the image on the wall. Everything on the top will be on top, everything on the bottom will be on the bottom. However, what is the left-hand side for you will be the right-hand side for someone facing the front of the piece of paper. Their left will be your right, and vice versus. However, their down will still be your down and their up will still be your up. Getting back to the image on the wall, what they see as the right side of the text will translate to be what you see as the left side of the image, and what they see as the left side of the text will be what you see as the right side of the image. In other words, the image on the wall is reverse relative to what someone looking at the piece of paper will see. However, down is still down and up is still up for both frames of reference. You could imagine the image is you somehow seeing through the piece of paper so your are seeing the outer surface from behind. The mirror is similar, what is on your left side is also on the left side of the image in the mirror. What is on your right side also is on the right side of the image in the mirror. What is your bottom will still be seen as the bottom in the mirror. What is your top will still be what is on the top in the mirror. However, since you are looking at it like it really exists, you think what is on your left in the image should be what is on someone's right who is behind the mirror. But it isn't, so you percieve it as being reversed. However, the imaginary person standing behind the mirror that you are basing your perceptions on would still see the same things as being up and down as you, since your are merely seeing a projected image.
I am sorry, this is not very clear. Hopefully it is at least intelligable, but I wouln'dt count on it. And I know the ray optics are all screwed up, but if I have no idea how to explain the ray optics of mirrors without pictures.
Order of Kilopi
What you are saying is that the issue is one of perception. That's right, because what a mirror really does is reverse front to back, the rest is up to you. But given that, why then do we so clearly perceive it as reversed left to right? I think the answer must have something deeply to do with our psychology. To test this, actually write the word "mirror" on a piece of paper, and then watch very closely what you do when you "hold it in front of a mirror". When you get what I'm saying, then ask, does the orientation of our eyes have anything to do with this? I really don't know why our eyes are oriented left and right. Is this more easily done biologically, and perhaps this gives rise to the psychology, or does the psychology guide the biology? Or is there no connection at all, and other factors are all that matter?
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Our brain does not change the actual image appearing on our retina (at least not significantly). What the brain does is tell us it is backwards, when it is really just a projection. I'm sorry, someone else should probably explain this, I am probably just confusing you and anyone else reading this.
Order of Kilopi
You have not tried the experiment I suggested. Again, do it, and watch your every move very carefully. When did the writing get reversed?
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Take that piece of paper with a word on it, and go stand in front of a mirror. First hold the paper facing yourself so that you can read it. Then turn the paper (rotation around a verticle axis) to face the mirror. Look at the image in the mirror and you see the word is now backwards. Now start over, by turning the paper back around so it is facing yourself so that you can read it properly. This time, flip the paper (rotation around a horizontal axis) to face the mirror. Look at the image and note that this time the word is simply upside down but not backwards.
It's not really the mirror that causes the reversals. It's what you do to the paper to make it face the mirror.
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If I hold the edge of a card with "Mirror" on it up to the mirror, with writing parallel to the mirror, and look down on it, the writing is reflected top-to-bottom. With the writing perpendicular, it's truly reflected left-to-right. With the card parallel to the mirror, it's neither.
The writing is inverted in each case, of course, but that's only conventionally described as left-to-right.
Order of Kilopi
I thought about this once a long time ago, and came up with an explanation (the same as the one with the paper) that satisfied me.
If I walk away from a mirror, and want to turn to look in the mirror, there are two ways to do it.
(1) I can turn around horizontally. In this case, the right and left will be reversed, but up and down will be OK.
(2) I can stick my head on the floor and put my feet in the air (do a headstand). Then, the right and left will be OK, but up and down will be reversed.
Of course, you won't be able to read letters on your t-shirt either way, because it's equivalent for the left and right to be switched and the top and down to be OK, or for top and down to be reversed while left and right are OK. To be able to read the letters on your t-shirt, you'd have to reverse both (or keep them the same). Which is impossible, because if you twist sideways AND turn on your head, you'll be facing away from the mirror again.
As above, so below
Order of Kilopi
Yes, you create the illusion yourself, in how you physically hold the card up to the mirror. So why do we all do it that way? Would a dolphin (if it had opposible thumbs...)? Is this related to our eyes being left and right, or just the way we interact with the world? I'm trying to figure out exactly where the illusion comes from.
Order of Kilopi
I agree with the spirit of what you're saying, but ironically, if you actually do that experiment you'll see that the letters on your shirt will still look reversed left to right! Because your head will be upside down too. Try it when you're not wearing the shirt...Originally Posted by Jens
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KenG & Jens got me thinking: It might be the handstand thing.
It has nothing to do with perception, or the mirror.
(at the equator) suppose you are facing north, and you are looking at the card facing south. To get the mirror image, you have to face the card north. To get it there, you do one of two things: Turn the card left/right, or spin the card upside down. To see the face in a mirror, you will have had to turn the card around first. Get it?
Edit to add: To turn around, you will tend do the left/right thing rather than the handstand thing since the body is more adapted to gravity than any other directional force. Therefore turning something around is more naturally left/right rather then up/down.
Order of Kilopi
Yes, this is exactly what I'm driving at. It's all in how you present the card to the mirror. But that's what I mean by perception, in the sense that you perceive there being one obvious way to present the card to the mirror, when in fact there is one other semi-reasonable way, and an infinite number of rather arbitrary ways to do it.
This is what I'm trying to get at, the reasons behind the perception. I think you may be right that the direction of gravity is crucial. But would a dolphin find it so obvious which way to present it? Perhaps so, for no creature likes to be upside-down for long. How about an astronaut who has been in orbit for a year?
Last edited by Ken G; 2005-Nov-10 at 04:31 PM.
Order of Kilopi
I think it has more to do with our being accustomed to a two-dimensional world. Turning around and facing a different direction is very natural, and the most sensible way to head off in a new direction, rather than flipping over and walking on your hands. And we probably turn the paper around the way we do because that's the way you'd have to turn it around if you want somebody standing next to you to be able to read it.
It's possible that our eyes are side by side rather than up and down because of the fact that , but I think it's more likely just because we happened to turn out bilaterally symmetric, which is probably more or less coincidental (it's probably pretty easy for bilateralness to happen genetically, and it leads to a reasonably balanced body, which is probably good for movement in all kinds of environments).
It's possible that a dolphin might see it differently, but I doubt it. Dolphins do sometimes turn around by flipping over, but they still stay preferentially upright. I'm fairly certain that a year in orbit wouldn't be sufficient to change the perceptual habits of a lifetime for an astronaut. A child who had been raised in zero gravity, though? Or maybe it's hard-wired, and you'd have to find a species that normally lives in zero gravity...
Order of Kilopi
Ah, I see, you are bringing in the fact that the way we would pass the paper is related to the type of translations that map one person's location into another's. One does wonder what it would be like if we were all floating around with arbitrary orientations in a zero-g environment.
That's what I was wondering as well.
Last edited by Ken G; 2005-Nov-10 at 08:18 PM.
Order of Kilopi
I'm not so sure I would call it an illusion. Printed material held up to a mirror is just hard to read, no doubt about it.
Here's another approach, lay the book down in front of the mirror as if you were reading the book, and try to read the text in the mirror--it still reads from your left to your right, but it's upside down.
Order of Kilopi
Yes, that's an equally valid way to "hold it to the mirror", good point. I actually had to do this to see if the letters were reversed or not-- hard to picture (they're not)! That would be a fun way to argue to your friends that mirrors reverse top to bottom. You might win a bet!
Order of Kilopi
There is a fairly famous experiment (famous in introductory psychology courses anyway) where a group of people put on goggles made of prisms such that their view of the world is upside down. After a while (less than a few days) of stumbling around, they become accustomed and they seem to perceive the view as rightside up. But that is not the amazing part (to me). When they take off the goggles, everything is upside down to them! And they have to be re-accustomed again.
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Order of Kilopi
I've heard of that, and I think it's amazing. I've always wanted to try that to see what it's like.I'll accept the general point that our perceptual habits can change in some ways pretty quickly. But I'll bet that an astronaut that's been in free fall for a long time would still turn the paper around side to side, rather than top to bottom.
Order of Kilopi
That's quite interesting hhEb09'1 -- it connects with one thing I've often wondered. Because of the nature of lenses, everything that comes into our optical nerve is already upside down-- we are wearing those goggles all the time! So does this mean that the ground is really above us right now? In other words, have we learned to accomodate our vision to our other senses such that we flip the image in our minds, or is it that we've accomodated our other senses to our vision such that we "feel" our feet as being below us, even though they are in fact above us! The experiment proves at least that the former possibility is physiologically possible, but it does not rule out that the latter is also physiologically possible. So I argue that at the moment, we really don't know which is true!
Order of Kilopi
Well, it's definitely psychological. All you have to do is draw a diagram showing how the light reflects "vertically" and how it reflects "horizontally" and you'll see that it does the same thing both ways.
We live in a three-dimensional world. Physically turning an object around any of the three axes will result in a reversal in two of the three dimensions. The mirror reverses only a single dimension, which is why the image is backwards - it produces a view of the object that is physically impossible in the real world.
It is quite normal, physically, for our view of objects or people to be rotated around the vertical axis, which reverses front-and-back and left-and-right. It is most decidedly abnormal for the view to be rotated around one of the horizontal axes, which would reverse either front-and-back and up-and-down or left-and-right and up-and-down.
So when we look in a mirror, we are presented with a view that is reversed front-and-back but neither up-and-down nor left-and-right. The "real world" tells us that a front-and-back reversal is always accompanied by a left-and-right reversal. Since the mirror image does not include that left-and-right reversal, we interpret the left-and-right as being wrong.
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Instead of a piece of paper, put the word on a piece of clear plastic. Hold it out in front of you. Look at the plastic sheet, now look in the mirror. Both images are the same. Turn it backwards and it is backwards both directly and in reflection.
When you turn it end for end, you put the letter that was on the left over to the right end - as seen from your perspective. If you turn it over around the horizontal axis, then it appears upside down rather than left/right inverted.
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The mirror doesn't reverse the image left to right, and it doesn't reverse it top to bottom. The right side is still on the right, and the left side is still on the left. The top is still on the top, and the bottom is still on the bottom. The only thing that changes is the direction your nose points.
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We make the mistake because our bodies are vertically symmetrical, but not horizontally symmetrical. We are used to seeing people facing us, and mental left-right reversals. If you stood in front of a mirror, while a 2nd person lay on his side (on the floor by your feet), facing the mirror, you would say that he has been flipped vertically.
If we were horizontally symmetrical creatures, we would make the mistake of saying that the mirror flips us horizontally. If we were asymmetrical creatures, we wouldn't make any mistake at all.
(It gets really interesting when you consider non planar mirrors. Now, concave mirrors reverse all three axes (after the focal point). The nose points backwards, the feet point up, and the left hand points right. Since all three axes are flipped, it's the same as rotating the image. The size of the image also changes.)
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Fascinating, thanks for sharing that. Now I want to try it. You've got my curiosity running.
Order of Kilopi
What about the fictitious "space" on the other side of the mirror? It must play a major role in our perception.
Order of Kilopi
The focused image our eyes see is upside down on the backs of our retinas. Our brains turn the images right-side up.
Order of Kilopi
Hey, did those prisms just flip the image vertically, or did they actually rotate it 180 degrees?
If the former, the brain's adjustment is even more amazing...
Order of Kilopi
A lot of people are just looking at the OP, and then giving their input, which is leading to repetition. What we are on now is, how is our brain controlling our perception such that we flip the paper left-to-right when we hold it to the mirror. Some good answers have been offered on that. My claim is still outstanding-- since our eyes flip the image of the world around us, we don't really know if the ground is above us, and we flip the perception from our other senses, or if it's below us and we flip the visual image! Do we?
Order of Kilopi
I think we do, since we know how the optics of the eye work, and we know that the image is in fact flipped from what it's like in the real world. I think if you did the switch you're suggesting, you'd just be redefining up and down to each mean the opposite.
Order of Kilopi
No, what I'm saying is, we know that the perceptions of our eyes are inverted with respect to our other senses, yes? But why is it obvious then that the brain flips the visual image to agree with the other senses, instead of the converse? We know the brain is capable of flipping the visual image, due to the experiment described by hhEb09'1. But we don't know it isn't capable of the same feat with the other senses. It is possible that only people who have been blind since birth know which way is up!
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