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

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: <a href="http://www.bbc.co.uk/archive/feynman/" rel="nofollow">http://www.bbc.co.uk/archive/feynman/</a><p>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.

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.

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.

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

Mirrors don't flip anything.<p>Oversimplification (ignores angle of light from object observed to eye), but here:<p>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.<p>Now imagine photons leaving all your parts, traveling to the mirror, and reflecting straight back.<p>That's all.

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

The technical-interview answer:<p>"Mirrors don't reverse left and right. They reverse in and out."<p>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.<p>Works every time.

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).

Assuming a mirror's face is its front:<p>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.<p>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.

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.<p>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.<p>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.

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.<p>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!<p>Now stick the bag on your foot. Look, mirror you's bag foot and your bag foot both point the same way!<p>Now stick the bag on your head. Look, mirror you's bag head is in the dark!<p>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.

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.

It's because mirrors are magic.<p>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.

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.<p>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?

To go along with fun mirror-things to think about, how about this one:<p>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.)

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.)

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<p>so i guess this question is like asking: why is 1 + 1 always equals 3 but no 4?<p>such question can have two meanings:<p>1: person reqly thinks that 1+1 = 3, answare is in the mindset of person that asked question<p>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)

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.

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.

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

actually it does flip up and down, you just have to put the mirror on the ceiling. or on the floor.<p>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.

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

Imagine you are a starfish.

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.

In a mirror you see what the mirror is seeing.

nice try, feynman

When looking at a mirror a human will imagine a clone has gone over to the other side, <i>rotated around the vertical axis</i>, then <i>inverted along the horizontal axis parallel to the mirror</i>. 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.)<p>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.<p>The question reveals a lot about the biases of the the person doing the asking.

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