Why are our images reversed from left to right and not up and down in a mirror?
November 6, 2000 12:25 PM   Subscribe

Why are our images reversed from left to right and not up and down in a mirror? You can get mirrors that give a 'true' reflection.
posted by john (20 comments total)
Hey!!! I had stuff to do today! Now I can't get this out of my head!

"it reverses front and back along an axis perpendicular to the mirror."

Oh. Ya..... And then ..... uh..... AAAHHHH!!!!
posted by y6y6y6 at 1:04 PM on November 6, 2000

Because someone looking in the mirror is making an
implicit comparison:

Let's say that Mr. X has a wing instead of a right arm.
Now, let's imagine he has an identical twin, Mr. Y, who
also has a wing instead of a right arm.

Let's say that Mr. X is looking at a mirror and Mr. Y. When
he looks at Mr. Y, he sees a man with a wing on Mr. Y's
right side, but Mr. X's left side. When he looks at the
mirror, he sees a man with a wing on Mr. X's right side.
Making the mental calculation, Mr. X notes that if the
mirror man really existed, the wing would be on the
mirror man's left side.

All this is to say that the mirror actually DOESN'T
reverse right and left OR up and down, but the
implicit comparison is to a situation where you
stare at your twin, and in this case, left and right
ARE reversed.

This is much easier to understand in mathematical
terms: imagine you stand at (3,0,0) in the xyz
coordinate system, where z is up and the floor is
the xy plane. A reflection across the yz plane is
equivalent to a rotation of 180 degrees around the
z-axis and a reflection about the xz axis. That
xz axis reflection turns left to right and vice versa,
but doesn't change up and down.
posted by UrineSoakedRube at 1:07 PM on November 6, 2000

Uh, because our eyes are side-by-side? Seems a lot quicker to explain, that way.
posted by dhartung at 1:12 PM on November 6, 2000

No, it has nothing to do with our eyes being side by
side. A cyclops would also notice that left and right
are "reversed" in a mirror.
posted by UrineSoakedRube at 1:36 PM on November 6, 2000

"A cyclops would also notice that left and right
are "reversed" in a mirror."

And he'd keep bumping into it too, since he'd lack depth perception.

posted by smcbride at 1:39 PM on November 6, 2000 [2 favorites]

Simple explanation: Left and right aren't reversed in a mirror. If you move your left hand, the image in the mirror moves the hand that to you is on the left side. It's that simple.
posted by daveadams at 2:12 PM on November 6, 2000

I just lost all respect for philosophers. A mirror reverses left/right but not up/down... what kind of garbage thinking is this? The mirror doesn't reverse anything. It reflects. Have any of these PhD philosophers ever taken a basic course in physics? Apparently not. This is so far away from a paradox that I'm nearly beside myself.

Wait. Here's an even trickier one that I just thought up: elevators allow a person to travel up/down, but almost never left/right over large lateral distances! Oh my! Why is that? How can that be? Isn't up/down analogous to left/right? I hope I haven't vexed the genius philosophers with this one.

When Holt wrote, "This question might seem foolish at first," he was right. Where he goes wrong is in continueing his analysis and trying to build a case against it being foolish. He never does. Right up till the end of the article, the question seems foolish. Rightly so.

It's particularly funny that it took these guys 14 years to come up with the perpendicular axis theory. Seriously. Read a damn physics text book. Better yet, apply some regular scientific thinking to the problem and trace where the rays of light go when they reflect. An 8-year-old could conduct that thought experiment successfully. Philosophers are still struggling with it. That truly is funny.
posted by Ronny at 2:34 PM on November 6, 2000 [1 favorite]

It's interesting that MeFi people are quick to proffer a simple explanation, yet apparently it's a problem that has been "vexing philosophers for at least half a century now." You guys should go teach those philosophers a thing or two.
posted by gluechunk at 2:36 PM on November 6, 2000

Amusing isn't it. Though I am curious about those true mirrors, even though you only have to take a picture of yourself to get around the mirror effect.
posted by john at 2:40 PM on November 6, 2000

Also, if a true mirror and a regular mirror face each other, does the world blow up?
posted by gluechunk at 2:43 PM on November 6, 2000

Even worse (and from the same writer) we have a "proof" that the End is Nigh. What's most annoying is that they selected this as one of their ten best articles of the decade.
posted by EngineBeak at 2:47 PM on November 6, 2000

Well, if you tilt your head to the side then it is reversed up/down.

crazy, man. like... duuuuude.
posted by amanda at 4:10 PM on November 6, 2000

Hey, wait a minute ... "Why are our images reversed from left to right and not up and down in a mirror?" ... this isn't meant as some cryptopolitical comment, is it?
posted by EngineBeak at 6:27 PM on November 6, 2000 [1 favorite]

Richard Feynman gives the best explanation why this "phenomenon" occurs, in my opinion: Neither X nor Y (height or width) are transformed in a mirror. Z (depth) is transformed by -1. X and Y aren't transformed at all, are there?

Imagine this: your body is shrinking from your belly button, ever shrinking, ever shrinking... until you reach depth 0. Continue shrinking (expanding in the negative directon) until you reach -1 * z. There's your completed transformation.

Sure, it's simple, but it isn't easy. I think it's ridiculous that some people belittle those who don't understand it immediately. I remember struggling with this a lot when I was a little boy... I don't think that I could have fathomed a science experiment to figure this out when I was eight years old.

I must be a retard.
posted by SilentSalamander at 6:59 PM on November 6, 2000 [1 favorite]

Gravity. Honest. It's just that powerful... 9.8m/s²
posted by hobbes at 7:38 PM on November 6, 2000

'Mirror's don't reflect', 'Neither X nor Y are transformed'... there's something missing from all these comments. Take a magazine, and hold it up to the mirror. We don't even need to talk about left and right - the writing is flipped horizontally, not vertically. a capital 'B', symmetrical vertically (roughly) but not horizontally, is reversed. A capital 'A', symmetrical horizontally but not vertically, looks ok.

Now take the magazine again. Look at the cover. Now turn it towards the mirror about the (long) horizontal axis, not the vertical one that you almost certainly rotated it around the first time. Voila, the reflection is reversed up-down, not left right.
posted by ledge at 2:21 AM on November 7, 2000

On the Physiscs of it all: http://library.thinkquest.org/22915/reflection.html

From jkottke's site, where he talked about this subject a while ago (scroll down).
posted by ericost at 5:47 AM on November 7, 2000

Alright, I've made a diagram, as simple as possible, to demonstrate. It may take a little imagination to extrapolate to the level of 'picture' rather than 'x' and '=', but I'm sure anyone can do it.
posted by EngineBeak at 9:09 AM on November 7, 2000

This just shows what doofuses these philosophers are. I can hardly believe it.

By the way, you can make your own "true mirror" for a lot less than $195.

Just take two regular mirrors and connect them at the edges, and make the angle between them 90 degrees. To use the mirror, look towards the connected edges (should be a vertical line in the middle of your face). Voila.

And then go back to using your regular mirror, because you're used to yourself looking the "reversed" way, and you realize you look disconcertingly odd in a "true" mirror.
posted by beth at 9:09 AM on November 7, 2000

Ronny demonstrates a lack of reading comprehension that would scandalize even GW Bush (with smugness to match). He seems to think that the question posed in the article is about the physics of reflection. It isn't. It has nothing whatever to do with optics, so if you're tempted to refer people to a "basic physics textbook", you're definitely on the wrong track.

Indeed, the philosophers are the only ones making sense here.

The question is why certain natural language terms which describe direction in the xy plane -- viz., "left"/"right" and their equivalents in all other human languages -- have denotations that are reversed in the mirror image, whereas certain terms describing direction along the z axis -- viz., "up"/"down" -- do not.* Given that you can easily define a language whose direction terms have completely different properties, why do all human languages encode this particular convention? This linguistic question obviously has nothing to do with the physics of light. The answer lies in neuroscience and -- ultimately -- evolutionary biology.

*For example, if I face the mirror and put my right hand up, it's at least somewhat natural to say: "my imaginary counterpart" put "his" left hand up; whereas it's not at all natural to say: "my imaginary counterpart" put his hand down. Why those particular linguistic conventions? Rough answer: given the way humans are shaped (bilaterally symmetric etc), adoption of these conventions conferred a selective advantage. If humans had evolved in different circumstances, we might have had different conventions.
posted by johnb at 12:05 PM on November 7, 2000

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