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Why Do Mirrors Flip Things Horizontally but Not Vertically? Here's the Physics
- deleted 6y ago[deleted]
- dhimes 6y agoThey don't. They keep the right on the right, the left on the left, the top on the top and the bottom on the bottom. I never quite got why this was a paradox tbh.
- arbitrage 6y agoYup. Mirrors reflect front to back. It's the struggle between accepting as literal what you've always been told, and the data being reported by your own senses, mitigated by language and cliché.
- rvieira 6y agoAgree. I don't mean to belittle people that get confused by this (many other things perplex me), but this always seemed very intuitive to me.
- gowld 6y agoIntuitively, why does a mirror-image appear to be flipped left-to-right compared to a photograph?
- afwe 6y agoIsn't it the photograph that is flipped? After all if I'm in front of a mirror my left hand appears to the left and the right hand appears to the right. But on a photograph it's the opposite.
- qayxc 6y agoIntuitively just consider the difference between the two: What DO you see? In case of the mirror, you see what's BEHIND you, because the stuff in front of you is blocked by the mirror, no? And just to be clear, by "you" I mean your eyes, and in particular the direction your eyes are looking at. Now in contrast a photograph never shows what's BEHIND the camera (now that would be kind of funky, don't you think?), but - just like your eyes - only captures what's in front of the lens. And there's your difference.
- rvieira 6y agoI don't know how to "explain" intuition, but usually I imagine not myself but some kind of geometric shape in front of a mirror (say a square with the edges labelled). If I try to visualize how light bounces, it is intuitive that in a mirror I'm seeing the light bouncing from mirror -> me in the proper way and in a photo, I'm looking at the shape directly with a camera (I'm where the mirror was).
- krastanov 6y agoThe paradox exists because when you ask a person "why it flips horizontally but not vertically" most people can not respond. Usually they can not even begin discussing the premise of the question. Notice how even in your comment you did not really attack the premise, just gave a parallel explanation that skips talking about the premise. This does not really help, because as with most paradoxes, the issue is exactly with the premise. The linked video by Physics Girl does a great job at explaining the paradox away.
- dhimes 6y agoI've found that explanation to be pretty good, as a matter of fact. The paradox seems to arise because from a psychological effect of "projecting" the directions with respect to the image "looking" back at you. Once you get people to stay in their reference frame (left, right, top, bottom above) they see their mistake.
- jbaker1 6y agoProb the best comment here
- IshKebab 6y agoThey mirror things forward-to-back.
- MichaelZuo 6y agoHow would it be possible to construct a mirror that does flip vertically?
- cassianoleal 6y agoLie on your side in front of one.
- badRNG 6y agoI suppose a concave mirror would.
- Yetanfou 6y agoTake 2 flat mirrors, set them edge to edge at an angle of 90°, mirror side facing into the angle. Position the mirrors in front of you with the edge horizontal and both mirrors angled at 45° towards you, i.e. look into the inner corner made by the 2 mirrors. The image you see in that corner will be flipped vertically. The same trick can be used to create a horizontal-flipping mirror, simply by positioning the mirrors so that the edge is oriented vertically. You can make one of these by sticking two pieces of mirror in an inside corner in a room somewhere.
- kace91 6y agograb a piece of paper with text on it. place it in your hands as if you were about to read it. Now flip it so a mirror in front of you can see it. The text is right to left, isn't it? Now notice that, in order to "show it to the mirror", you applied an horizontal rotation to the paper. Try placing the piece of paper as it was at the beginning, so it's readable to you again, and then "show it" again to the mirror, but this time flipping it with a vertical flip. There you go, the text is left to right but upside down. Vertical flip! You're probably thinking "yeah but the original is upside down too" to which I would reply, yes, and in the first case the original was also right to left from your position - you just coudn't see it because the paper is opaque. writing in a piece of glass , you can see it left to right in the mirror when it's left to right in real life.
- wlesieutre 6y agoA normal mirror already does flip things vertically just as much as it does horizontally, which is to say not at all. This about your expectations of what a person (or thing) facing you looks like. Normally when someone turns to face you they'll spin around while staying in a head-upward orientation. That puts your right hand and their right hand on opposite sides because they've spun around, while your reflection's right hand stays on the same side as your own because the mirror doesn't flip anything. If you instead expect the other person to turn to face you by doing a hand stand, then you can say "Why is my reflection's head on top?! It should be on the bottom! The mirror has flipped it vertically!"
- cassianoleal 6y agoThis is terrible physics! ~~First, the Z axis is commonly used to represent height rather than width.~~ Second, the mirror is not flipping anything. It's just reflecting things in a straight line. This is easily demonstrated by holding a sign with a transparent background in front of the mirror. The image you see on the back of the sign aligns with the one you see on the mirror. A person in front of the sign is able to read the words properly because, relative to you (and the sign) they are flipped. Their right eye is in front of your left eye and vice-versa. The mirror gives the impression of a flipped image precisely because it's not flipping anything! Edit: It's already been pointed out twice to me that Z for height is not in any way a convention. I never thought it was, which is why I used the phrase "commonly used" instead. Turns out that doesn't seem to be the case either, regardless of my personal experience so this is an acknowledgement of that mistake.
- happytoexplain 6y agoI think it's pretty reasonable to say the image is flipped, but it's flipped along the 3rd axis: Front-to-back.
- TuringTest 6y agoExactly. The image in a mirror is physically symmetrical to the direct image from objects in the real world, with the mirror itself as the plane of symmetry. I find the front-to-back explanation to give a better intuition than the typical "but left and right are relative to your body", which only serves to explain the paradox of "left-right are reversed but up-down ar not", but not the fact that everything looks flipped.
- AnimalMuppet 6y agoWell, it flips front-to-back. But this side of the mirror, we can't physically do that, so it's not in our brain's list of expected possibilities. We can, however, rotate ourselves 180 degrees. And because we're bilaterally symmetrical, our brain finds it easy to interpret the front-to-back flip as a 180 degree rotation. But that leaves left-to-right weird, because what happened wasn't actually a 180 degree rotation.
- ffpip 6y agoMirrors don't 'flip' things. Also, z axis is generally used as the height.
- wtetzner 6y agoIn computer graphics z is generally used as the depth. See OpenGL's z-buffer.
- zupreme 6y agoTheir conclusion is inaccurate and may be disproven simply by leaning oneself over to a 90 degree angle, or greater, while leaving the mirror and the text source stationary. Meh. Onward.
- wtetzner 6y agoThe conclusion is wrong? The conclusion is that mirrors "flip" (for lack of a better term) front-to-back.
- philjohn 6y agoI love the Feynman video on this - https://www.youtube.com/watch?v=msN87y-iEx0 https://www.youtube.com/watch?v=msN87y-iEx0
- throwaway738 6y agoThe first 2 sentences of the article, literally: "Mirror image is a tricky thing, but it's pretty clear when you look at words in a reflection, that mirrors flip things horizontally rather than vertically. Or, at least it seems that that's the case." Majority of HN comments: "A mirror doesn't flip anything" The "At least it seems that's the case" clearly suggests that there isn't an actual flip. But ask a regular person whether a mirror flips that is what they will say. The article addresses both the common wisdom and the actual science.
- tgb 6y agoAbsolutely, this is a pretty solid explanation and HN (and similar places) always have the bizarre take on explaining of paradoxes to shout "It's not a paradox!" Paradoxes are about appearing, at first thought, to be impossible, not it being literally impossible. In this case, it's the two observations: 1) A mirror is rotationally symmetric so should treat left/right the same as up/down. 2) A piece of paper with writing held up to a mirror reads right-to-left instead of left-to-right. But it does not read bottom-to-top. This asks for an explanation - one that the article gives. Why do so many here feel the need to say that since this has an explanation, it is not a paradox? Outside of fiction, every paradox has an explanation and, moreover, we're discussing an article that gives that explanation. But: a mirror does flip something, it flips in-and-out. Mirror images are not the same.
- plorkyeran 6y agoThe (perhaps overly) charitable take on "It's not a paradox!" responses is that it's something which didn't appear impossible or weird to that responder, and the article failed to make the case for why there's even something that needs an explanation.
- tedunangst 6y agoI'm curious to know what percentage of commenters who know exactly how mirrors work can perform simple tasks like assembling a puzzle with the same speed only looking through a mirror.
- koalafied 6y agoUse east/west instead of left/right and you’ll see there’s no flipping. Left and right are defined with respect to the person, and so they are different for the person and their image. East and west (or up and down) are defined with respect to environment, and they’re not flipped.
- saddestcatever 6y agoThank you! So simple, so beautiful of an explanation.
- TuringTest 6y agoAre you sure? How do you explain the sun in the mirror casting shadows in the opposite direction in the mirror than in the real world? ;-)
- TuringTest 6y agoIf you use east/west as constants, you will find that the mirror is flipping north and south. No matter what convention you use to define relative positions between objects, the mirror is physically creating a symmetric image of those, switching distances from closest to farthest from the glass.
- koalafied 6y agoThat’s a good point, I had not thought of this dimension.
- teddyh 6y ago> And why, when you raise your right hand, does your mirror-self raise its left hand, but it still moves it up rather than down? The words “up” and “down” are absolute, but the words “left” and “right” are relative to you. If we instead of “left” and “right” use the words “south arm” and “north arm”, then the mirror suddenly does not “flip” anything. This is a strong clue that the apparent paradox is conceptual, not real.
- deleted 6y ago[deleted]
- iforgotpassword 6y agoHmmm. Just made me think that you probably couldn't explain that "paradox" to most 6yo who just about learned to tell left and right apart. In my experience they will consistently get left and right hands of others wrong when facing them, because they always treat those relative to themselves. So the mirror doesn't flip anything to them.
- dragontamer 6y ago> Hmmm. Just made me think that you probably couldn't explain that "paradox" to most 6yo who just about learned to tell left and right apart. A typical 6-year old can say what their left-hand is. But if you tell them to point to MY left hand, they will instead point to my right hand. It takes additional effort and training to teach young children how to see from other people's perspective.
- YetAnotherNick 6y agoThis seems reasonable explanation on first glance, but on further thinking it seems to miss something. Assume 'down arm' as leg, and 'up arm' as head, relative to the person who has the arm just like left and right arm. Even then the mirror doesn't flip up and down arm, but only flips left and right.
- hammock 6y agoagreed. As another commenter mentioned, what a mirror does is not flip left-right, but front-back. (notice front-back are still relative terms, so parent comment argument is moot)
- lordfoo 6y agoMirrors flip things front-to-back
- withinboredom 6y agoPhysics Girl explains this in the video.
- azundo 6y agoPut a mirror on the ceiling and it will "flip" things vertically and not horizontally.
- quietbritishjim 6y agoThere are really two "flips" that are often talked about with a mirror: 1. It flips things in the direction perpendicular to the mirror. If you're facing a normal wall-mounted mirror then that means it flips your "forward" into your "backwards". This is a real physical effect that no one would normally dispute. 2. It appears to flip things left to right, and most people (before mirror-flipping enlightment) would reasonably believe this. But it's a human psychological effect, not a real physical one. This article is about the second point. You're talking about the first point. The placement of the mirror is a red herring: even with a ceiling mounted mirror, if you show a piece of paper to it by putting it above your head in the natural unenlightened way and look up, you'll find the text is still flipped left to right. In fact it maybe even emphasises the illusion because you can turn to face any direction (and without thinking you'll turn the piece of paper too of course) and still see the same effect! (BTW I upvoted your comment because it's not the only comment to post this mix up - but it seems to be the first - so I think it deserves a bit of visibility for others thinking the same thing.)
- gowld 6y agoA Silence "Alert" article about a 5 year old video that is a remake of a 37-year old video. All of them fine in their own right, but... Sometimes I think we're nearing the end of human advancement because we've built up so much knowledge that we're forgetting as fast as we learn.
- gvx 6y agoI think it's just the "today's lucky 10 000" effect (https://xkcd.com/1053/ https://xkcd.com/1053/).
- zuminator 6y agoThe image in the mirror appears to have reversed chirality. Reversed chirality is simply that, it doesn't have a preferential direction of horizontal or vertical. But our environment has a preferential direction in that we orient ourselves with respect to the Earth's surface, and the horizon is, well, horizontal. Our body is constructed with vertical bilateral symmetry which means our "halves" are horizontally oriented when in a normal standing or sitting position. All of this causes us to perceive and define reversed chirality in terms of handedness, left or right. Things that we perceive with a left-to-right orientation appear as right-to-left if chirally reversed. If we were spherical beings with limbs and eyes poking out at various angles from our bodies, living in microgravity on a space vessel, we would just perceive the reversed chirality in our mirrors as "enantiomorphic" and not assign a spurious L/R directionality to it.
- abnry 6y agoThe confusion ultimately comes from a nebulous understanding of "flip". This is what we mean when we say our mirror image is flipped: We imagine the mirror represents an extension of our normal 3D world and rotate our body around the z-axis until our head and legs line up. That's we interact with our 3d world: we translate and rotate objects. However, this isn't the only way to "flip" (i.e. rotate back). It seems like the only way because it is the only way to rotate the mirror image onto something that aligns with the non-mirror reality. The head must align with the head and the legs must align with the legs, and there is only one rotation that makes it work. But now imagine you had some sort of isle of man flag [1] type body. That is, your arms remain, but your feet and your head turn into an arm. For lack of better term, call your "head-arm" upper-handed and call you "leg-arm" lower-handed. Now when you look at your freakish mirror image, there are TWO ways you can rotate your the mirror image to correspond in some way with your body. You can rotate as usual around the z-axis, but you can also rotate around the y-axis. In that case, your left and right arms are not flipped, but your upper-arm and lower-arm are flipped. That is, the whole confusion occurs because humans are symmetric in one dimension and non-symmetric in another. We are: ==Y== X=O=X ==Z== As opposed to ==Y== X=O=X ==Y== [1] https://en.wikipedia.org/wiki/Flag_of_the_Isle_of_Man https://en.wikipedia.org/wiki/Flag_of_the_Isle_of_Man
- mokus 6y agoI always felt like the explanations people give are way too complicated. The simple version, to me at least, is this: A mirror doesn’t flip left to right. It flips front to back. YOU flip left to right when you turn things to the mirror.
- 0-_-0 6y agoYou're in a room with your friend. You look at a sign on his forehead saying "AOM". There's a mirror on the wall. You can see your friend in the mirror, the sign on his forehead is now saying "MOA". Nobody turned anything to the mirror, the whole scene is stationary. Why is it "MOA" instead of "VOW"?
- notthemessiah 6y agoYou can make a mirror flip something vertically, just put the mirror on the ground.
- paulsutter 6y agoFalse - Mirrors don't reverse ANYTHING. Look in a mirror. Raise your right hand, THE HAND ON THE RIGHT GOES UP. There is no reversal, it's a myth. When you stand facing a real person, THATS the reversal. If they raise their left hand, you see it on the right. Because THEY are reversed. They are facing the opposite direction.
- gvx 6y agoDid you watch the video? Mirrors reverse front and back.
- 0-_-0 6y agoEver tried to read text in a mirror? Why is the word AMBULANCE flipped horizontally on ambulances and not vertically? Why is ƎƆ ИA⅃UᙠMA and not ∀WB∩Г∀ИCE or ƎƆ N∀˥∩𐐒W∀?
- edflsafoiewq 6y agoWrite AMBULANCE on a piece of paper. Now turn the paper around so the word faces away from you. Hold it up to a light and you can see that the word is now flipped horizontally. Now face a mirror: the image is projected onto the mirror with no change, so you see the exact same thing you saw when it held it up the light. IOW the mirror is inessential. We can ask the same question about just a transparent piece of paper.
- 0-_-0 6y agoThat's because you flipped the paper horizontally not vertically. If you flipped it vertically it would be flipped verically. But you didn't flip the ambulance.
- paulsutter 6y agoThe text isnt reversed, you're seeing it from the back. The leading A and the trailing E are in the same right to left position that they are in the real sign. There is no reversal.
- deleted 6y ago[deleted]
- retube 6y ago> Is your mind blown? Yep, ours too. Hhhmm, no. I can't even understand the supposed problem / paradox here?
- pfortuny 6y agoThis is so dependent on the system of coordinates that it is not even wrong.
- JoeAltmaier 6y agoStand facing a mirror. Hold an open book up so you can read it. Now turn it to face the mirror. The text is backward! Wait. How did you turn the book? Right-to-left? Well, that's why its 'backward'. Try it again, but this time flip the book vertically to face the mirror. Now the test is not flipped right-to-left, but top-to-bottom. It's nothing to do with 'physics'. Its just perception.
- 0xdeadbeefbabe 6y agoAnd why don't the show vampires?
- dboreham 6y agoThis is probably why a cat doesn't recognize itself, even as another animal, in a mirror.
- mdoms 6y agoJust link the YouTube video. The article is nothing but a summary of the video, and a very incomplete summary at that.
- oxipital 6y agoPlenty of ads though. I mean there's a video ad playing while the Youtube ad is playing The internet is fucking lazy anymore.
- 51Cards 6y agoRichard Feynman explaining it is wonderful: https://www.youtube.com/watch?v=6tuxLY94LXw https://www.youtube.com/watch?v=6tuxLY94LXw
- speakeron 6y agoMy way of thinking of it is like this: imagine your eyes were located above and below your belly button (on a vertical axis). What would be 'flipped' when you look in the mirror?
- Justsignedup 6y agoOr as Shafer the dark lord puts it: Because mirrors are lazy.
- nendroid 6y agoNothing is flipped on the xy plane. The flipping happens on the Z-axis. Your reflection is a translation of yourself across the Z-axis into the mirror then a flip along the depth axis so that your reflection faces you. When you look at your face, both your left eye and your reflected left eye are still to your left relative to you. Same with your right eye. Your chin is the same. Your chin is below your actual nose and your reflected chin is still below your actual nose. All positions are PRESERVED along the xy plane. What flipped is the Z axis. Imagine you wearing face as a mask then flipping it inside out along the z-axis so that the face mask is now looking at you. That is the flipping that is going on... all other positional coordinates are preserved. People who say there is no "flipping" going on are completely wrong. There is a flip: on the z-axis.
- gnonio 6y agoWe are trained on rotated mirrors in the vertical axis (Y), seldom we see them rotated on the horizontal axis (X). If you go to an hotel with mirrors on the ceiling or on the floor, the reflections are all flipped vertically.
- oxipital 6y agoI'm more amazed by the ad filled lazy wrapping of a youtube video in a web page to basically remonetize the video. Fuck the internet's gotten lazy.
- desertlounger 6y agoI always thought it is because our eyes are arranged horizontally and not vertically...
- dsr_ 6y agoIf the effect persists when you close one eye, that's not the answer.
- anothercommnt 6y agoThis variable can be removed by covering one eye :-)
- deleted 6y ago[deleted]
- GrumpyNl 6y agoNothing is flipped, its a mirror.
- nvr219 6y agoThis "article" is just writing out the YouTube someone else made, and tacks on 28 uBlock Origin hits and 12 Privacy Badger hits. Here's the link to the very good original YouTube from Physics Girl: https://youtu.be/vBpxhfBlVLU https://youtu.be/vBpxhfBlVLU
- dr_zoidberg 6y agoTahnk you, this also bugged me. Why link the article when it's just a rehashing over a great video? Wouldn't it be better to just link the video? Whatever the article adds, it's very little. We're talking about an under 4 minute video that you can very quickly watch, no furth explanations needed (and the article provides none at that).
- nvr219 6y agoYou're welcome Dr. Zoidberg.
- edanm 6y agoThat is a great video. I actually have a hard time even thinking of this as a paradox - not in the sense of "oh it's obvious", just in the sense that when I first heard about this, it had honestly never occured to me that there was something weird going on, and it took me a while to even understand what the question was. (Sidenote - this is often a metric for a good question!) This video is an excellent explanation of what's going on, tying it into other common experiences people have, in a way that makes a lot of connections clear.
- rbanffy 6y agoDid we really have a discussion about this? Seriously?
- nwellnhof 6y agoA mirror does flip up and down if you rotate 180 degrees around the x axis and end up standing on your head behind the mirror. It's just that we typically associate rotation with the y axis.
- justincredible 6y agoThe linked video is a proper superset of the article and explains it better.
- Vanit 6y agoI don't understand why there is any confusion about this or needs any explanation at all that's not related to photons.
- nullc 6y agoMirrors reverse front and back. If you imagine your face painted on a surface, like a mask, then look at it from behind, that is essentially what the mirror is doing. I think this is the best intuition, among other things it gives the right intuition for how circularly polarized waves reflect.
- Doxin 6y agoA mirror doesn't mirror left-to-right, it mirrors front to back. The clue being that you're looking at a reflection of your face in a mirror, not a reflection of the back of your head. Of course since nothing but a mirror can do that in day-to-day life most people expect a mirror to do a 180 degree rotation instead, leading to this confusion.
- proto-n 6y agoMirror flips front and back actually. You are facing north towards the mirror. Point south, reflection points south. Point up, reflection points up. Point north, reflection poins south. Left/right flips because it is calculated relative to up and forward, the latter of which has flipped in the mirror.