15 ms·

A mirror flips left and right, but not up and down

152 points by ochekurishvili 15y ago

deleted 15y ago
[deleted]
jrp 15y ago
The book "Good and Real" opens with this thought experiment and has a good discussion.
extension 15y ago
Hold a book in your hands while standing in front of a mirror. Now turn the book around so you can read it in the mirror. The text is backwards. But how did you rotate the book? Sideways? If you had flipped it over vertically, the text would be upside down instead of backwards. You are the one who reversed the book, not the mirror.
- dpark 15y ago
  Exactly. Stand in front of a mirror holding a sheet of transparency (such as you would use for an overhead projector) in front of you so that you can read it. Look at the transparency and note that you can read the words. Now look at the transparency in the mirror and note that you can still read the words. If you rotate the transparency (left/right, up/down, corner/corner, whatever) so that you can no longer read it directly, you'll see that the same rotation has occurred in the mirror.
bauchidgw 15y ago
i never understood the question: how could anybody ever even think that a mirror flips left and right? there is no emperical way to observe this behaviour (with a single flat mirror).
- andrenotgiant 15y ago
  This is one of those Feynman questions where understanding the question requires a certain way of thinking that discards common sense. Similar to his "Why is the wall not see-through?" question. The way I think about is: "When you are looking at a mirror, and you blink your left eye, mirror-you blinks his right eye. But when you are laying on your side looking at a mirror, and you blink your top eye, mirror you blinks his top eye."
  - bauchidgw 15y ago
    still don't get it: when i blink my left eye, mirror me blinks his left eye. maybe this question would make sense, if i would see myself as the center of the universe, all creation and all there is - but there is a pretty high chance that i ain't. (and even then i'm not sure if this question would make sense)
    - Jtsummers 15y ago
      The question itself stems from confusion about what's being seen. It's not uncommon, especially with children, for people to become slightly confused while using a mirror. For instance, on seeing some smudge on their left cheek trying to wipe it from their right cheek. Once they start moving of course the error becomes obvious. Their initial understanding of the image is really what the image would be if you had a camera facing you and then used a video display as your mirror.
      - omaranto 15y ago
        I have this problem all the time! I even shave by touch and then check in the mirror whether I've missed a spot rather than shave while looking in the mirror because I can't avoid being confused about this. I can, while looking at the mirror, reason about which way I need to move my hand to get at a certain spot on my face's reflection, but it's much faster to stop looking at the mirror and let (correct) instincts kick in.
        narcissus 15y ago
- InclinedPlane 15y ago
  If there were a person who looked like a version of you who was "flipped" left/right they would look exactly like the mirror version of you. However, it's equally valid to consider such a "mirror-person" as being flipped top/bottom or inverted front to back. However, these are much less intuitive transforms.
  - bauchidgw 15y ago
    >If there were a person who looked like a version of you who was "flipped" left/right they would look exactly like the mirror version of you. if the left/right switched person would stand where mirror me is standing, loooking in the direction where mirror me is starring, he would not look like mirror me, he would be switched left/right.
    - randallsquared 15y ago
      We tend to assume that the real world is not switched left-right, because when we're in that position, we don't feel switched left-right.
JonnieCache 15y ago
The various feynman "fun to imagine" videos, one of which is linked by the answerer, are amazing. I advise you to watch all of them as soon as you can. They're all up on youtube as linked on stackexchange, or you can see them on the BBC's site here: http://www.bbc.co.uk/archive/feynman/ http://www.bbc.co.uk/archive/feynman/ Also, feynman's explanation is much more intuitive and more easily understood than the one given on stackexchange. As you might imagine. Also it comes replete with the inevitable feynman tales of intellectual dickswinging that we have all come to know and love.
- monochromatic 15y ago
  > intellectual dickswinging Yes... but not just intellectual.
  - JonnieCache 15y ago
    True. But fortunately that doesn't come up in the BBC interview. Also I don't think swinging is quite the right verb for that area of his pursuits.
    - Daniel_Newby 15y ago
      Feynman was a bit of a womanizer, for those who don't know. http://xkcd.com/182/ http://xkcd.com/182/
- hammock 15y ago
  Feynman says some interesting stuff but none of these explanations have yet satisfied me. Sure, I get it that a mirror technically does not rotate the image, it does the near/far thing instead, flips the nose, flips the north/south, however you want to put it. But, this the fact remains: I hold an object in front of me (there is no mirror anywhere). IF I want to see/simulate what the object will look like in a mirror, I rotate it LEFT TO RIGHT (on the vertical plane, around the line that is parallel to my body no matter whether I am standing or laying down). I cannot rotate it any other way and get the same effect. WHY is that? EDIT: Whoops, you guys are all right, I'm an idiot. Here I am sitting at my desk with quite literally nothing but bisymmetric objects around me- for a second there I went a little crazy. Thanks for setting me straight
andrenotgiant 15y ago
nice try, feynman
InclinedPlane 15y ago
Mirror's don't flip left and right. We implicitly compare a mirror image to the original by rotating the object, and naturally we maintain top/bottom orientation, this gives the appearance of "flipping" the object left to right. However, if you use vertical rotation to super-impose the object on its mirror image you'll see that it seems to have been flipped top-to-bottom instead.
- onemoreact 15y ago
  Another way of thinking about it is mirrors swap what is near and far instead of rotating. If your twin was standing on the other side of glass without rotation you would see their back instead of your front. If the mirror is above you then you see your head. If you twin is standing above you then you see their feet. Flipping thins inside out sounds strange, but it also works. If you take a mask with the same face on the inside and outside and look behind the mask you see the same thing as what you see in the mirror excluding depth perception issues. Finally, you can think of mirrors as swapping both left and right and top to bottom. If your twin is rotated so their right hand is touching your right hand and their left hand is touching your left hand then their feet are up in the air. (Easiest to visualize with two action figures.)
  - InclinedPlane 15y ago
    This works for any plane through the object, a mirror will appear to "swap" the object along that plane into its mirror image. Left/Right, top/bottom, front/back, even NNW/SSE.
- jws 15y ago
  The little players on a foosball table probably think mirrors flip up and down, since that is their normal method of turning around.
  - dasil003 15y ago
    Is that true or does their mind explode because they see a figure that can only exist wearing the other color shirt?
adyus 15y ago
It's because mirrors are magic. On the serious side, this would be an interesting question to ask a child. We can all reason the answer from our knowledge of physics and the rules of the world in general (where there are no cool explanations for this), but a child has no such constraints. Their imagination runs free, and they fill in the blanks from their own knowledge of the world, which may include magic, super powers, or whatever realities they came in contact with in their short life.
- omaranto 15y ago
  The heartless bastards that down voted you obviously haven't spoken to young children and heard their charming, clever and totally wrong explanations for things. If they had, they would probably agree it's an interesting question to ask them.
doyoulikeworms 15y ago
Assuming a mirror's face is its front: Photons bounce off the right side of you, strike the left side of the mirror, and then reach your eyes. Similarly for the left side. Photons bounce off the top side of you, strike the top side of the mirror, and then reach your eyes. Similarly for the bottom side.
- sixtofour 15y ago
  I think this explanation is based on the (erroneous) tendency of people to think of what they see in the mirror as themselves turned around. Look at the desk in front of you. The right hand drawer is near your right hand. The left hand drawer is near you left hand. Now why did you just call the right side of the mirror, which is near your right hand, as the left side?
  - doyoulikeworms 15y ago
    It assumes that the mirror's face is its front, making the right side of the mirror near your left hand. I only described it this way to reduce word count. We can say something similar using your more precise terms-- "near left/right hand". I did not describe the mirror "turning you around". If it was yourself turned around, you'd see photons from your doppelganger's right hand near your left (as is the case when you face someone). Since it's a reflection, which does not turn anything around (as you said), your image of your left hand is near your actual left hand. To wrap it all up-- the reflection of your head is near your head, and your feet is near your feet. This is why you are reversed left-right, and not top-down in a flat mirror.
skeltoac 15y ago
My answer (Andy) explains how it's really the photograph that is flipped. When we say the mirror flips the left-right directions, we're accidentally referring to the absence of the familiar rotation of the photograph.
protothomas 15y ago
I find a nice way of look at it is to think of it as looking at the back of a two-dimensional 'slice' of what is in front of the mirror.
ruibai 15y ago
Skipping all the details, it's because you have a left eye and a right eye, not an up eye and a down eye.
- RickHull 15y ago
  The effect does not go away if you only have (or use) one eye.
  - david927 15y ago
    Turn the mirror 90 degrees and nothing happens. Turn your head 90 degrees and it flips up and down.
martincmartin 15y ago
Mirrors don't flip left and right, or up and down. They flip front and back.
- BenoitEssiambre 15y ago
  This is the first part of the answer. The second part is that, since we define the left and right on an object relatively to its front and back, flipping the front and back also swaps how left and right are defined. There is no confusion with top and bottom as they are not defined relative to front and back.
  - martincmartin 15y ago
    Absolutely true.
- qntm 15y ago
  This is the entire answer. It amazes me that people can write so much about this question without managing to get to this statement.
daimyoyo 15y ago
I think a more interesting question is how the rear view mirror works. What I mean is how does a perfectly clear image become slightly dimmer when you adjust the angle of the mirror? I've seen that since I was a child but I've never understood why it works.(I never got past calc based physics in college so if the answer is obvious I apologize.)
- dpark 15y ago
  I'm assuming you're talking about what happens when you flip the manual "dimming" switch on the mirror. I'm pretty sure you're looking into a mirror that actually has two reflections. One of them is already dim, and it's dominated by the brighter one so you don't normally see it. (Sometimes in the right light you can see both reflections at once.) The two reflections are offset, so that when the bright reflection is showing you the view out the rear window, the dim reflection is showing you a few degrees lower, probably the back seat. When you flip the dimming button, the mirror turns upward a few degrees, so that the dim reflection is now showing you the view out the rear window. You can't see the bright reflection any more because it's now showing you the roof inside the car, which is pretty dark in relation to the rear window. Now the "dim" reflection is actually bright enough to dominate. I assume that this is accomplished with some sort of double mirror, where the "bright" mirror is closer to you, and some light leaks through it to hit the "dim" mirror behind. These could be made into a single unit with no actual space between them. It just needs two layers of reflective material that are at slightly different angles.
  - chadgeidel 15y ago
    Or materials with different angles of refraction - which is what I always thought. (I don't actually know.)
- 7rurl 15y ago
  The bright reflection is the reflection off the silver backing of the glass in the mirror (i.e. the normal mirror reflection). The dim reflection is the reflection off the surface of the glass (the same as the reflection you see of yourself when you look at a window when it is dark outside and bright inside). Flipping the "switch" on the mirror simple changes the angle of the mirror by the exact amount needed to reflect the light from your rear window into your eye off either the silver surface or the glass surface.
losvedir 15y ago
To go along with fun mirror-things to think about, how about this one: Stand in front of a mirror. Note the lowest point on your body that you can see in the reflection, whether it's your waist or thighs, or whatever. Now, no matter how close you get to the mirror or how far you step back, that will always be the lowest point on your body in the reflection. (e.g., you'll never be able to see your toes, if you couldn't initially.)
- JonnieCache 15y ago
  LIES! I just tried this, the visible portion increased by about a 50cm when I stepped back about 3m. I guess this is because my big mirror isn't exactly flat to the wall. Come to think of it, it isn't even completely flat.
  - losvedir 15y ago
    Ha, good point. I neglected to include that the mirror must be flat.... I'm glad you tried it, though! My friend told me this and I just didn't believe it until I tried it for myself.
lambdapilgrim 15y ago
I will try my short explanation that I have been telling myself since the first time I heard this question. Imagine a mirror on the ceiling. From the same understanding that we are judging the image to be flipped left to right, we will now notice that the image is flipped top to bottom. Explanation: Imagine a stick without width whose image is reflected in the mirror on the ceiling. If we superimpose the image on the stick, haven't we flipped top and bottom?
mnemonicsloth 15y ago
The technical-interview answer: "Mirrors don't reverse left and right. They reverse in and out." Then tell the interviewer that only an idiot would look at a local coordinate frame and think it's a global one. Just what is he insinuating? This had better not be one of those "stress interviews" people talk about. There's no way in hell you're going to work for a company that will screw around with you on day one just to see how you'll react. Works every time.
Terretta 15y ago
Stop using the words "left" and "right" or "up" and "down". You're confusing yourself thinking about "right" hand or "left" hand. In mirror you's world, those words have different meanings. Instead, put a paper bag on one hand and stick your hands out. Look in the mirror. Your "bag" hand and mirror you's "bag" hand are both pointing the same way! Now stick the bag on your foot. Look, mirror you's bag foot and your bag foot both point the same way! Now stick the bag on your head. Look, mirror you's bag head is in the dark! But seriously, everything in the mirror is oriented congruently with this "side" of the glass. The appendages wearing a bag always match up. "Bag hand" and "not bag hand" match just like head and feet do.
mwerty 15y ago
Imagine you are a starfish.
sixtofour 15y ago
Mirrors don't flip anything. Oversimplification (ignores angle of light from object observed to eye), but here: Imagine standing in front of a mirror, both arms straight out. A photon leaves your left index finger, travels straight to the mirror, and is reflected straight back. Now imagine photons leaving all your parts, traveling to the mirror, and reflecting straight back. That's all.
- dsmithn 15y ago
  That doesn't answer the question, though. You have to explain why that means my right hand appears to be on my left side.
  - sixtofour 15y ago
    The question is "Why is it that when you look in the mirror left and right are flipped, but not the up and down?" When I look in the mirror I see my right hand exactly where it should be, on the right side of the mirror. If I approach the mirror and then touch it, my right hand touches the right hand image in the mirror, as do my forehead and toes respectively. Someone who asks this question, after understanding what's happening with the light, appears to expect that the left hand with the watch (for example) in the mirror should be on the same side of the mirror as the real left hand with the watch if the observer turned himself 180 deg (away from the mirror). That's a psychological phenomenon, not a physical one, and I don't know why we do that. One way to answer, then, is that mirrors don't flip anything, our brains do it, and erroneously.
ww520 15y ago
My high school history teacher asked this same question (yes, it is that old) and my answer at the time was: Left and right are assigned relative to where you are facing, while up and down are not regardless where you're facing. The same part of your body is reflected at the same side in the mirror (paint your cheeks with different colors; the mirrored cheeks have the same color on the same side as the normal). However, we CALL the cheek left or right depending on where we are facing. The mirrored image reverses the facing so its left and right are ASSIGNED that way, though it's the same physical cheek. Up and down are absolute regardless of where we face. A mirrored image only changes the "facing" aspect so up and down don't change.
- reitzensteinm 15y ago
  You know, that was exactly my answer as a teenager as well, and the answer I thought I knew when I clicked on this thread, but I don't think it's a sufficient explanation. Even if you assigned up and down as relative, no vertical flipping appears to take place. I believe InclinedPlane's answer [1] is much better. We implicitly perceive the reflection as having rotated as a human would, by turning around on his or her feet. However, if instead the reflection is perceived as having rotated vertically, with the human flipping over onto his or her head, the image is actually flipped vertically. So the answer is actually that the image in the mirror is either flipped horizontally or vertically, depending on one's perception of how the mirror image got there in the first place. While the local/global orientation explanation does show that there are no inconsistencies, and the lack of a relative orientation for 'up' clouds things a bit, it doesn't actually explain at all why we naturally look at the image and decide it's flipped horizontally. [1] http://news.ycombinator.com/item?id=2790792 http://news.ycombinator.com/item?id=2790792
  - ww520 15y ago
    Left and right has one frame of reference - facing. Up and down has another frame of reference - room ceiling/floor. Mirror inverses the facing frame of reference but not the room frame of reference. Left and right, up and down are just convention in our mind. We human use facing to assign left and right, and use ceiling and floor to assign up and down. There is no switching sides in the mirror in reality. You red hand (if you color it) is on the same side inside or outside of the mirror, just as your head and feet are on the same side inside or outside of mirror. We just assign a new left/right to the mirrored image because we see where the facing in the mirror is; however, our head is near the ceiling and feet are still on the floor so the room frame of reference hasn't changed, and thus up/down are labeled the same in the mirror. The point my history teacher trying to make via this exercise was to say most stuffs (in history particular) were relative in a frame of reference. It's the same event but through a different angle would be called different thing, when moved to a different frame of reference. The mirror exercise is the terrorist/freedom fighter argument in physical illustration.
slowcpu 15y ago
If your eyes were stacked one above the other, then the mirror would flip up and down.
- jackpirate 15y ago
  try shutting one eye and see if you still believe that
bluedanieru 15y ago
When looking at a mirror a human will imagine a clone has gone over to the other side, rotated around the vertical axis, then inverted along the horizontal axis parallel to the mirror. That last part is the perceived "flip" but it is a direct consequence of the imagined axis of rotation. If you imagine a being who is accustomed to a horizontal axis of rotation in their movement, you can see how they would imagine their clone going to the other side of the mirror, rotating around the horizontal axis, then inverting along the vertical axis parallel to the mirror. They would then ask why a mirror flips up and down, but not left and right, for they are accustomed to up and down switching after a rotation, where we are accustomed to left switching with right. (In fact I suspect we identify "up and down" as strongly with this invariant as we do with gravity.) Any being with freedom of action along some plane, who is accustomed to rotation around an axis perpendicular to that plane, will experience much the same thing: a rotation around some axis parallel to the mirror, followed by an inversion around the axis parallel to the mirror and perpendicular to the rotation. They will then perceive a "flip" along that axis of inversion. The question reveals a lot about the biases of the the person doing the asking.
Confusion 15y ago
It's too bad the question, repeated in the title, is phrased in such a way that it will perpetuate the myth that mirrors flip left and right. I wouldn't mind some editorializing so the title no longer claims the switch as fact.
majmun 15y ago
actually it does flip up and down, you just have to put the mirror on the ceiling. or on the floor. so to conclude; by definition of this question the result is fixed, you are allowed to fix y- axis but not x-axis when you are translating mirror image to real world image.
tintin 15y ago
In a mirror you see what the mirror is seeing.
majmun 15y ago
when you have mirror in front than, left and right are not changed on mirror image but front and back. including side mirror and bottom mirror it goes like this: Front miror: front/back:yes, top/down:no, left/right:no Side mirror: front/back:no, top/down:no, left/right:yes Bottom mirror: front/back:no, top/down:yes, left/right:no so i guess this question is like asking: why is 1 + 1 always equals 3 but no 4? such question can have two meanings: 1: person reqly thinks that 1+1 = 3, answare is in the mindset of person that asked question 2: person is just making sounds taht resembles those of human talk (like parrots) and this is not even question. but mimicing (common in nature)
princeverma 15y ago
isn't it simply because our eyes are in horizontal plane. If they would have been in vertical plane we would see upside down in mirror.
olavk 15y ago
If you stand on a mirror it does flip up and down.

152 points by ochekurishvili 15y ago

deleted 15y ago

[deleted]

jrp 15y ago

The book "Good and Real" opens with this thought experiment and has a good discussion.

extension 15y ago

Hold a book in your hands while standing in front of a mirror. Now turn the book around so you can read it in the mirror. The text is backwards. But how did you rotate the book? Sideways? If you had flipped it over vertically, the text would be upside down instead of backwards. You are the one who reversed the book, not the mirror.

dpark 15y ago
Exactly. Stand in front of a mirror holding a sheet of transparency (such as you would use for an overhead projector) in front of you so that you can read it. Look at the transparency and note that you can read the words. Now look at the transparency in the mirror and note that you can still read the words. If you rotate the transparency (left/right, up/down, corner/corner, whatever) so that you can no longer read it directly, you'll see that the same rotation has occurred in the mirror.

bauchidgw 15y ago

i never understood the question: how could anybody ever even think that a mirror flips left and right? there is no emperical way to observe this behaviour (with a single flat mirror).

andrenotgiant 15y ago
This is one of those Feynman questions where understanding the question requires a certain way of thinking that discards common sense. Similar to his "Why is the wall not see-through?" question. The way I think about is: "When you are looking at a mirror, and you blink your left eye, mirror-you blinks his right eye. But when you are laying on your side looking at a mirror, and you blink your top eye, mirror you blinks his top eye."
- bauchidgw 15y ago
  still don't get it: when i blink my left eye, mirror me blinks his left eye. maybe this question would make sense, if i would see myself as the center of the universe, all creation and all there is - but there is a pretty high chance that i ain't. (and even then i'm not sure if this question would make sense)
  - Jtsummers 15y ago
    The question itself stems from confusion about what's being seen. It's not uncommon, especially with children, for people to become slightly confused while using a mirror. For instance, on seeing some smudge on their left cheek trying to wipe it from their right cheek. Once they start moving of course the error becomes obvious. Their initial understanding of the image is really what the image would be if you had a camera facing you and then used a video display as your mirror.
    - omaranto 15y ago
      I have this problem all the time! I even shave by touch and then check in the mirror whether I've missed a spot rather than shave while looking in the mirror because I can't avoid being confused about this. I can, while looking at the mirror, reason about which way I need to move my hand to get at a certain spot on my face's reflection, but it's much faster to stop looking at the mirror and let (correct) instincts kick in.
      - narcissus 15y ago
InclinedPlane 15y ago
If there were a person who looked like a version of you who was "flipped" left/right they would look exactly like the mirror version of you. However, it's equally valid to consider such a "mirror-person" as being flipped top/bottom or inverted front to back. However, these are much less intuitive transforms.
- bauchidgw 15y ago
  >If there were a person who looked like a version of you who was "flipped" left/right they would look exactly like the mirror version of you. if the left/right switched person would stand where mirror me is standing, loooking in the direction where mirror me is starring, he would not look like mirror me, he would be switched left/right.
  - randallsquared 15y ago
    We tend to assume that the real world is not switched left-right, because when we're in that position, we don't feel switched left-right.

JonnieCache 15y ago

The various feynman "fun to imagine" videos, one of which is linked by the answerer, are amazing. I advise you to watch all of them as soon as you can. They're all up on youtube as linked on stackexchange, or you can see them on the BBC's site here: http://www.bbc.co.uk/archive/feynman/ http://www.bbc.co.uk/archive/feynman/ Also, feynman's explanation is much more intuitive and more easily understood than the one given on stackexchange. As you might imagine. Also it comes replete with the inevitable feynman tales of intellectual dickswinging that we have all come to know and love.

monochromatic 15y ago
> intellectual dickswinging Yes... but not just intellectual.
- JonnieCache 15y ago
  True. But fortunately that doesn't come up in the BBC interview. Also I don't think swinging is quite the right verb for that area of his pursuits.
  - Daniel_Newby 15y ago
    Feynman was a bit of a womanizer, for those who don't know. http://xkcd.com/182/ http://xkcd.com/182/
hammock 15y ago
Feynman says some interesting stuff but none of these explanations have yet satisfied me. Sure, I get it that a mirror technically does not rotate the image, it does the near/far thing instead, flips the nose, flips the north/south, however you want to put it. But, this the fact remains: I hold an object in front of me (there is no mirror anywhere). IF I want to see/simulate what the object will look like in a mirror, I rotate it LEFT TO RIGHT (on the vertical plane, around the line that is parallel to my body no matter whether I am standing or laying down). I cannot rotate it any other way and get the same effect. WHY is that? EDIT: Whoops, you guys are all right, I'm an idiot. Here I am sitting at my desk with quite literally nothing but bisymmetric objects around me- for a second there I went a little crazy. Thanks for setting me straight

andrenotgiant 15y ago

nice try, feynman

InclinedPlane 15y ago

Mirror's don't flip left and right. We implicitly compare a mirror image to the original by rotating the object, and naturally we maintain top/bottom orientation, this gives the appearance of "flipping" the object left to right. However, if you use vertical rotation to super-impose the object on its mirror image you'll see that it seems to have been flipped top-to-bottom instead.

onemoreact 15y ago
Another way of thinking about it is mirrors swap what is near and far instead of rotating. If your twin was standing on the other side of glass without rotation you would see their back instead of your front. If the mirror is above you then you see your head. If you twin is standing above you then you see their feet. Flipping thins inside out sounds strange, but it also works. If you take a mask with the same face on the inside and outside and look behind the mask you see the same thing as what you see in the mirror excluding depth perception issues. Finally, you can think of mirrors as swapping both left and right and top to bottom. If your twin is rotated so their right hand is touching your right hand and their left hand is touching your left hand then their feet are up in the air. (Easiest to visualize with two action figures.)
- InclinedPlane 15y ago
  This works for any plane through the object, a mirror will appear to "swap" the object along that plane into its mirror image. Left/Right, top/bottom, front/back, even NNW/SSE.
jws 15y ago
The little players on a foosball table probably think mirrors flip up and down, since that is their normal method of turning around.
- dasil003 15y ago
  Is that true or does their mind explode because they see a figure that can only exist wearing the other color shirt?

adyus 15y ago

It's because mirrors are magic. On the serious side, this would be an interesting question to ask a child. We can all reason the answer from our knowledge of physics and the rules of the world in general (where there are no cool explanations for this), but a child has no such constraints. Their imagination runs free, and they fill in the blanks from their own knowledge of the world, which may include magic, super powers, or whatever realities they came in contact with in their short life.

omaranto 15y ago
The heartless bastards that down voted you obviously haven't spoken to young children and heard their charming, clever and totally wrong explanations for things. If they had, they would probably agree it's an interesting question to ask them.

doyoulikeworms 15y ago

Assuming a mirror's face is its front: Photons bounce off the right side of you, strike the left side of the mirror, and then reach your eyes. Similarly for the left side. Photons bounce off the top side of you, strike the top side of the mirror, and then reach your eyes. Similarly for the bottom side.

sixtofour 15y ago
I think this explanation is based on the (erroneous) tendency of people to think of what they see in the mirror as themselves turned around. Look at the desk in front of you. The right hand drawer is near your right hand. The left hand drawer is near you left hand. Now why did you just call the right side of the mirror, which is near your right hand, as the left side?
- doyoulikeworms 15y ago
  It assumes that the mirror's face is its front, making the right side of the mirror near your left hand. I only described it this way to reduce word count. We can say something similar using your more precise terms-- "near left/right hand". I did not describe the mirror "turning you around". If it was yourself turned around, you'd see photons from your doppelganger's right hand near your left (as is the case when you face someone). Since it's a reflection, which does not turn anything around (as you said), your image of your left hand is near your actual left hand. To wrap it all up-- the reflection of your head is near your head, and your feet is near your feet. This is why you are reversed left-right, and not top-down in a flat mirror.

skeltoac 15y ago

My answer (Andy) explains how it's really the photograph that is flipped. When we say the mirror flips the left-right directions, we're accidentally referring to the absence of the familiar rotation of the photograph.

protothomas 15y ago

I find a nice way of look at it is to think of it as looking at the back of a two-dimensional 'slice' of what is in front of the mirror.

ruibai 15y ago

Skipping all the details, it's because you have a left eye and a right eye, not an up eye and a down eye.

RickHull 15y ago
The effect does not go away if you only have (or use) one eye.
- david927 15y ago
  Turn the mirror 90 degrees and nothing happens. Turn your head 90 degrees and it flips up and down.

martincmartin 15y ago

Mirrors don't flip left and right, or up and down. They flip front and back.

BenoitEssiambre 15y ago
This is the first part of the answer. The second part is that, since we define the left and right on an object relatively to its front and back, flipping the front and back also swaps how left and right are defined. There is no confusion with top and bottom as they are not defined relative to front and back.
- martincmartin 15y ago
  Absolutely true.
qntm 15y ago
This is the entire answer. It amazes me that people can write so much about this question without managing to get to this statement.

daimyoyo 15y ago

I think a more interesting question is how the rear view mirror works. What I mean is how does a perfectly clear image become slightly dimmer when you adjust the angle of the mirror? I've seen that since I was a child but I've never understood why it works.(I never got past calc based physics in college so if the answer is obvious I apologize.)

dpark 15y ago
I'm assuming you're talking about what happens when you flip the manual "dimming" switch on the mirror. I'm pretty sure you're looking into a mirror that actually has two reflections. One of them is already dim, and it's dominated by the brighter one so you don't normally see it. (Sometimes in the right light you can see both reflections at once.) The two reflections are offset, so that when the bright reflection is showing you the view out the rear window, the dim reflection is showing you a few degrees lower, probably the back seat. When you flip the dimming button, the mirror turns upward a few degrees, so that the dim reflection is now showing you the view out the rear window. You can't see the bright reflection any more because it's now showing you the roof inside the car, which is pretty dark in relation to the rear window. Now the "dim" reflection is actually bright enough to dominate. I assume that this is accomplished with some sort of double mirror, where the "bright" mirror is closer to you, and some light leaks through it to hit the "dim" mirror behind. These could be made into a single unit with no actual space between them. It just needs two layers of reflective material that are at slightly different angles.
- chadgeidel 15y ago
  Or materials with different angles of refraction - which is what I always thought. (I don't actually know.)
7rurl 15y ago
The bright reflection is the reflection off the silver backing of the glass in the mirror (i.e. the normal mirror reflection). The dim reflection is the reflection off the surface of the glass (the same as the reflection you see of yourself when you look at a window when it is dark outside and bright inside). Flipping the "switch" on the mirror simple changes the angle of the mirror by the exact amount needed to reflect the light from your rear window into your eye off either the silver surface or the glass surface.

losvedir 15y ago

To go along with fun mirror-things to think about, how about this one: Stand in front of a mirror. Note the lowest point on your body that you can see in the reflection, whether it's your waist or thighs, or whatever. Now, no matter how close you get to the mirror or how far you step back, that will always be the lowest point on your body in the reflection. (e.g., you'll never be able to see your toes, if you couldn't initially.)

JonnieCache 15y ago
LIES! I just tried this, the visible portion increased by about a 50cm when I stepped back about 3m. I guess this is because my big mirror isn't exactly flat to the wall. Come to think of it, it isn't even completely flat.
- losvedir 15y ago
  Ha, good point. I neglected to include that the mirror must be flat.... I'm glad you tried it, though! My friend told me this and I just didn't believe it until I tried it for myself.

lambdapilgrim 15y ago

I will try my short explanation that I have been telling myself since the first time I heard this question. Imagine a mirror on the ceiling. From the same understanding that we are judging the image to be flipped left to right, we will now notice that the image is flipped top to bottom. Explanation: Imagine a stick without width whose image is reflected in the mirror on the ceiling. If we superimpose the image on the stick, haven't we flipped top and bottom?

mnemonicsloth 15y ago

The technical-interview answer: "Mirrors don't reverse left and right. They reverse in and out." Then tell the interviewer that only an idiot would look at a local coordinate frame and think it's a global one. Just what is he insinuating? This had better not be one of those "stress interviews" people talk about. There's no way in hell you're going to work for a company that will screw around with you on day one just to see how you'll react. Works every time.

Terretta 15y ago

Stop using the words "left" and "right" or "up" and "down". You're confusing yourself thinking about "right" hand or "left" hand. In mirror you's world, those words have different meanings. Instead, put a paper bag on one hand and stick your hands out. Look in the mirror. Your "bag" hand and mirror you's "bag" hand are both pointing the same way! Now stick the bag on your foot. Look, mirror you's bag foot and your bag foot both point the same way! Now stick the bag on your head. Look, mirror you's bag head is in the dark! But seriously, everything in the mirror is oriented congruently with this "side" of the glass. The appendages wearing a bag always match up. "Bag hand" and "not bag hand" match just like head and feet do.

mwerty 15y ago

Imagine you are a starfish.

sixtofour 15y ago

Mirrors don't flip anything. Oversimplification (ignores angle of light from object observed to eye), but here: Imagine standing in front of a mirror, both arms straight out. A photon leaves your left index finger, travels straight to the mirror, and is reflected straight back. Now imagine photons leaving all your parts, traveling to the mirror, and reflecting straight back. That's all.

dsmithn 15y ago
That doesn't answer the question, though. You have to explain why that means my right hand appears to be on my left side.
- sixtofour 15y ago
  The question is "Why is it that when you look in the mirror left and right are flipped, but not the up and down?" When I look in the mirror I see my right hand exactly where it should be, on the right side of the mirror. If I approach the mirror and then touch it, my right hand touches the right hand image in the mirror, as do my forehead and toes respectively. Someone who asks this question, after understanding what's happening with the light, appears to expect that the left hand with the watch (for example) in the mirror should be on the same side of the mirror as the real left hand with the watch if the observer turned himself 180 deg (away from the mirror). That's a psychological phenomenon, not a physical one, and I don't know why we do that. One way to answer, then, is that mirrors don't flip anything, our brains do it, and erroneously.

ww520 15y ago

My high school history teacher asked this same question (yes, it is that old) and my answer at the time was: Left and right are assigned relative to where you are facing, while up and down are not regardless where you're facing. The same part of your body is reflected at the same side in the mirror (paint your cheeks with different colors; the mirrored cheeks have the same color on the same side as the normal). However, we CALL the cheek left or right depending on where we are facing. The mirrored image reverses the facing so its left and right are ASSIGNED that way, though it's the same physical cheek. Up and down are absolute regardless of where we face. A mirrored image only changes the "facing" aspect so up and down don't change.

reitzensteinm 15y ago
You know, that was exactly my answer as a teenager as well, and the answer I thought I knew when I clicked on this thread, but I don't think it's a sufficient explanation. Even if you assigned up and down as relative, no vertical flipping appears to take place. I believe InclinedPlane's answer [1] is much better. We implicitly perceive the reflection as having rotated as a human would, by turning around on his or her feet. However, if instead the reflection is perceived as having rotated vertically, with the human flipping over onto his or her head, the image is actually flipped vertically. So the answer is actually that the image in the mirror is either flipped horizontally or vertically, depending on one's perception of how the mirror image got there in the first place. While the local/global orientation explanation does show that there are no inconsistencies, and the lack of a relative orientation for 'up' clouds things a bit, it doesn't actually explain at all why we naturally look at the image and decide it's flipped horizontally. [1] http://news.ycombinator.com/item?id=2790792 http://news.ycombinator.com/item?id=2790792
- ww520 15y ago
  Left and right has one frame of reference - facing. Up and down has another frame of reference - room ceiling/floor. Mirror inverses the facing frame of reference but not the room frame of reference. Left and right, up and down are just convention in our mind. We human use facing to assign left and right, and use ceiling and floor to assign up and down. There is no switching sides in the mirror in reality. You red hand (if you color it) is on the same side inside or outside of the mirror, just as your head and feet are on the same side inside or outside of mirror. We just assign a new left/right to the mirrored image because we see where the facing in the mirror is; however, our head is near the ceiling and feet are still on the floor so the room frame of reference hasn't changed, and thus up/down are labeled the same in the mirror. The point my history teacher trying to make via this exercise was to say most stuffs (in history particular) were relative in a frame of reference. It's the same event but through a different angle would be called different thing, when moved to a different frame of reference. The mirror exercise is the terrorist/freedom fighter argument in physical illustration.

slowcpu 15y ago

If your eyes were stacked one above the other, then the mirror would flip up and down.

jackpirate 15y ago
try shutting one eye and see if you still believe that

bluedanieru 15y ago

When looking at a mirror a human will imagine a clone has gone over to the other side, rotated around the vertical axis, then inverted along the horizontal axis parallel to the mirror. That last part is the perceived "flip" but it is a direct consequence of the imagined axis of rotation. If you imagine a being who is accustomed to a horizontal axis of rotation in their movement, you can see how they would imagine their clone going to the other side of the mirror, rotating around the horizontal axis, then inverting along the vertical axis parallel to the mirror. They would then ask why a mirror flips up and down, but not left and right, for they are accustomed to up and down switching after a rotation, where we are accustomed to left switching with right. (In fact I suspect we identify "up and down" as strongly with this invariant as we do with gravity.) Any being with freedom of action along some plane, who is accustomed to rotation around an axis perpendicular to that plane, will experience much the same thing: a rotation around some axis parallel to the mirror, followed by an inversion around the axis parallel to the mirror and perpendicular to the rotation. They will then perceive a "flip" along that axis of inversion. The question reveals a lot about the biases of the the person doing the asking.

Confusion 15y ago

It's too bad the question, repeated in the title, is phrased in such a way that it will perpetuate the myth that mirrors flip left and right. I wouldn't mind some editorializing so the title no longer claims the switch as fact.

majmun 15y ago

actually it does flip up and down, you just have to put the mirror on the ceiling. or on the floor. so to conclude; by definition of this question the result is fixed, you are allowed to fix y- axis but not x-axis when you are translating mirror image to real world image.

tintin 15y ago

In a mirror you see what the mirror is seeing.

majmun 15y ago

when you have mirror in front than, left and right are not changed on mirror image but front and back. including side mirror and bottom mirror it goes like this: Front miror: front/back:yes, top/down:no, left/right:no Side mirror: front/back:no, top/down:no, left/right:yes Bottom mirror: front/back:no, top/down:yes, left/right:no so i guess this question is like asking: why is 1 + 1 always equals 3 but no 4? such question can have two meanings: 1: person reqly thinks that 1+1 = 3, answare is in the mindset of person that asked question 2: person is just making sounds taht resembles those of human talk (like parrots) and this is not even question. but mimicing (common in nature)

princeverma 15y ago

isn't it simply because our eyes are in horizontal plane. If they would have been in vertical plane we would see upside down in mirror.

olavk 15y ago

If you stand on a mirror it does flip up and down.