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

http://physics.stackexchange.com/questions/8227/a-mirror-flips-left-and-right-but-not-up-and-down

by ochekurishvili • 15 years ago

152 80 15 years ago

physics.stackexchange.com

jrp• 15 years ago

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

extension• 15 years 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.

dpark3• 15 years 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.

bauchidgw4• 15 years 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).

andrenotgiant2• 15 years 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."

bauchidgw4• 15 years 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)

InclinedPlane3• 15 years 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.

bauchidgw4• 15 years 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.

JonnieCache3• 15 years 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/

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• 15 years ago

> intellectual dickswinging

Yes... but not just intellectual.

JonnieCache3• 15 years 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.

hammock• 15 years 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

ars• 15 years ago

Rotating the object will not get you the same view as in the mirror. You would need to flip it inside out.

ldite• 15 years ago

I don't know about you, but if I turn a picture-postcard around its vertical axis, left-to-right, all I see is the blank back of the picture. (Likewise if I rotate it around it's horizontal axis.)

What you're actually saying is if I put the picture on transparent acetate (or whatever) and rotate it, then look at it through its back, I see the same as I would in the mirror. Which is essentially the same as reversing the depth of the object (when you think about it.)

And there's nothing privileged about the Left-to-right rotation. When you face the mirror and hold the picture up (facing the mirror, away from you), you chose to rotate it around its vertical axis to face the mirror (thus swapping left and right). If you chose to rotate it around its vertical axis, top-to-bottom, to face the mirror and away from you then you get the other situation.

dpark3• 15 years ago

Rotating an object left-to-right most certainly won't show you what it looks like it a mirror. It shows you the back of the object (assuming that the front of the object was previously facing you).

Here's an experiment to demonstrate that left-to-right rotation is not what a mirror does.

1. Take an object and hold it so that the front is facing you (pick a side and call it the front if there's not an obvious "front").

2. Rotate the object left to right so that you are looking at the back.

3. Write your name on the back of the object (the side currently facing you).

4. Rotate the object right-to-left so that the front is once again facing you.

5. Go to a mirror and hold the object so the front is facing you.

6. Look at the object in the mirror.

Does the object in the mirror look the same as when you did the left-to-right rotation? No. You can see the back of the object, but your name is backwards. What you're seeing in the mirror is not a left-right rotation. That's an explanation that your brain applies to the situation, but it's not what actually happens.

A similar experiment is to hold a coffee mug and stand in front of a mirror. Grasp the mug by the handle with your right hand. Look in the mirror. Where is the handle? It's still on your right. The coffee mug in the real world has the handle on your right, and so does the coffee mug in the reflection. There is no left/right reversal.

Edit: You're not an idiot. The mirror's behavior is simple, but it's also very non-intuitive. If it were intuitive, people wouldn't ask the question.

colanderman• 15 years ago

It's psychological. You could instead rotate the object head-over-heels to "simulate" reflection, and the object would then appear upside-down, but have left and right in the same position as on a mirror. However things don't normally move head-over-heels but rather by rotating left/right (thanks to gravity) so psychologically, we assume the object must have rotated left/right to get on the other side of the mirror, thus appearing reversed in that direction.

anigbrowl• 15 years ago

It doesn't flip left to right. It flips front to back. Hold something in front of a mirror; you > thing > mirror > thing > you.

It reorients things 'on the vertical plane' parallel to your body because your eyes are perpendicular to your vertical orientation and your brain is used to processing things that way. Put a mirror on the floor or the ceiling and mess about with some geometric solids for a while, you'll start to think differently about it. Alternatively, get some dark safety goggles, a laser, and a bunch of small mirrors on adjustable fixtures.

kenjackson• 15 years ago

The video says, "Not available in my area"! I don't think they realize I'm in the USA. The U-S-A! Everything is available to me. How could anyone let this happen?!

omaranto4• 15 years ago

That's surprising: I thought YouTube's unit of granularity was the country. I'm in the US too and can see the video. I guess this means it's available in some parts of the country and not others. Hurray for being in Boston, then.

numlocked• 15 years ago

It's funny - that series of videos is exactly the sort of video that would make someone a YouTube star today.

In the same way a teenage girl gains a following after making videos of how to apply makeup, I could easily see Feynman becoming a similar YouTube phenomenon had he lived now and released videos like this on a regular basis.

Always fun to imagine people from another era who you know would have been MASSIVE on social media (another one: Robert Townsend -> Twitter).

andrenotgiant2• 15 years ago

nice try, feynman

InclinedPlane3• 15 years 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• 15 years 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.)

InclinedPlane3• 15 years 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• 15 years ago

The little players on a foosball table probably think mirrors flip up and down, since that is their normal method of turning around.

dasil0032• 15 years ago

Is that true or does their mind explode because they see a figure that can only exist wearing the other color shirt?

khafra• 15 years ago

It would also be easier to see with an object that had near-perfect vertical bilateral symmetry but no horizontal bilateral symmetry; the opposite of a human in a normal posture.

omaranto4• 15 years ago

You could try with a human head turned sideways.

Knacker_Hughes• 15 years ago

The way I see it, the mirror does flip up and down, at least by the measure of the same experiment used to prove that it flips left and right.

To prove that it flips left and right, imagine a vertical pole between you and the mirror. The pole has a card attached to it with the front facing the mirror. The two dots, red and blue are painted on the card and you can see their reflection in the mirror.

In the reflection, the blue dot is on the right and the red dot is on the left.

If you rotate the pole so that is facing you and you can now see the front of the card you see that the blue dot is really on the left and the red dot on the right.

This proves that the mirror flips left and right.

To prove that it flips up and down, imagine the same pole in front of you with the card facing the mirror, but this time the pole is horizontal.

in the reflection you see that the blue dot is at the top and the red dot is at the bottom.

If you rotate the horizontal pole so that the card is now facing you, you see that the blue dot is really at the bottom and the red dot at the top.

gnosis• 15 years ago

"If you rotate the pole so that is facing you and you can now see the front of the card you see that the blue dot is really on the left and the red dot on the right.

This proves that the mirror flips left and right."

All it proves is that you've rotated the card. You've done the flipping, not the mirror.

Now, without doing any rotation to the card, if the card is seen from the perspective of the mirror, the blue dot is on the left, and the red on the right.

Depending on which perspective you're looking at an object from, one side will be on your left, and the other on your right. But no flipping takes place, unless you either move yourself to a different perspective, or rotate the object. A mirror does neither. It just reflects back exactly what's in front of it.. so you see whatever's on your right on your right, and whatever's on your left on your left. There is no flipping.

Natsu• 15 years ago

Or you can just put a mirror on the ceiling and look into it for a visual demonstration of it working the "other" way.

adyus• 15 years 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.

omaranto4• 15 years 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.

doyoulikeworms2• 15 years 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.

sixtofour3• 15 years 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?

doyoulikeworms2• 15 years 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• 15 years 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• 15 years 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• 15 years 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• 15 years ago

The effect does not go away if you only have (or use) one eye.

david927• 15 years ago

Turn the mirror 90 degrees and nothing happens. Turn your head 90 degrees and it flips up and down.

martincmartin2• 15 years ago

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

BenoitEssiambre• 15 years 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.

martincmartin2• 15 years ago

Absolutely true.

qntm• 15 years 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• 15 years 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.)

dpark3• 15 years 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• 15 years ago

Or materials with different angles of refraction - which is what I always thought. (I don't actually know.)

7rurl• 15 years 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.

losvedir2• 15 years 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.)

JonnieCache3• 15 years 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.

losvedir2• 15 years 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• 15 years 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• 15 years 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• 15 years 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• 15 years ago

Imagine you are a starfish.

sixtofour3• 15 years 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• 15 years 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.

sixtofour3• 15 years 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.

ww5202• 15 years 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• 15 years 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

ww5202• 15 years 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• 15 years ago

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

jackpirate• 15 years ago

try shutting one eye and see if you still believe that

bluedanieru• 15 years 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• 15 years 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.

majmun2• 15 years 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• 15 years ago

In a mirror you see what the mirror is seeing.

majmun2• 15 years 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• 15 years 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• 15 years ago

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

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

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