A mirror flips left and right, but not up and down • Hacker News Board

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

#11

Post by bauchidgw » Thu, Jul 21, 2011, 7:15 PM UTC

andrenotgiant wrote: post #9

bauchidgw wrote: post #5

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).

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 blink…

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)

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

#12

Post by monochromatic » Thu, Jul 21, 2011, 7:16 PM UTC

JonnieCache wrote: post #6

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 imag…

> intellectual dickswinging

Yes... but not just intellectual.

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

#13

Post by adyus » Thu, Jul 21, 2011, 7:17 PM UTC

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.

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

#14

Post by JonnieCache » Thu, Jul 21, 2011, 7:20 PM UTC

monochromatic wrote: post #12

JonnieCache wrote: post #6

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 imag…

> intellectual dickswinging Yes... but not just intellectual.

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.

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

#15

Post by bauchidgw » Thu, Jul 21, 2011, 7:20 PM UTC

InclinedPlane wrote: post #10

bauchidgw wrote: post #5

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).

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.

>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.

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

#16

Post by onemoreact » Thu, Jul 21, 2011, 7:21 PM UTC

InclinedPlane wrote: post #8

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.

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.)

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

#17

Post by jws » Thu, Jul 21, 2011, 7:26 PM UTC

InclinedPlane wrote: post #8

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.

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

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

#18

Post by [deleted] » Thu, Jul 21, 2011, 7:48 PM UTC

bauchidgw wrote: post #11

andrenotgiant wrote: post #9

Earlier quoted context omitted.

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 blink…

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)

[deleted]

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

#19

Post by InclinedPlane » Thu, Jul 21, 2011, 7:51 PM UTC

onemoreact wrote: post #16

InclinedPlane wrote: post #8

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.

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…

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.

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

#20

Post by Jtsummers » Thu, Jul 21, 2011, 7:54 PM UTC

bauchidgw wrote: post #11

andrenotgiant wrote: post #9

Earlier quoted context omitted.

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 blink…

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)

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.