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

~~~
hammock
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

~~~
dpark
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.

------
InclinedPlane
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.

~~~
Knacker_Hughes
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
_"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.

------
extension
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
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.

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

~~~
BenoitEssiambre
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
Absolutely true.

------
sixtofour
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
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
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.

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

~~~
david927
Turn the mirror 90 degrees and nothing happens. Turn your head 90 degrees and
it flips up and down.

------
mnemonicsloth
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.

------
bauchidgw
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
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
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
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
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
I had my tongue pierced a few years back and even now (not that I do it very
often), when looking in a mirror to put the ball back on the 'stick' (I don't
know what it's called!) I drive myself nuts.

In one hand I'm trying to hold/'point' the 'stick' and in the other hand, not
only am I trying to get the ball to the end of the stick, but at the same time
trying to rotate it so that the hole in the ball lines up with the end of the
stick. Everything points, moves and rolls totally against what I think it
should be doing.

I, like you, find it easier to just do away with the mirror and try and work
it out myself.

Having said that, this whole conversation has made me really _think_ about the
mirror and so I'm hoping that next time, it won't be so hard!

------
doyoulikeworms
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
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
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.

------
ww520
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
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>

~~~
ww520
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.

------
Terretta
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.

------
Confusion
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.

------
adyus
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
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.

------
lambdapilgrim
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?

------
losvedir
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
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
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.

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

------
majmun
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)

------
skeltoac
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
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.

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

------
majmun
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.

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

------
mwerty
Imagine you are a starfish.

------
princeverma
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.

------
tintin
In a mirror you see what the mirror is seeing.

------
andrenotgiant
nice try, feynman

------
bluedanieru
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.

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

~~~
jackpirate
try shutting one eye and see if you still believe that

