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

#41

Post by dpark » Thu, Jul 21, 2011, 9:43 PM UTC

extension wrote: post #4

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.

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.

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

#42

Post by lambdapilgrim » Thu, Jul 21, 2011, 9:47 PM UTC

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?

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

#43

Post by RickHull » Thu, Jul 21, 2011, 9:55 PM UTC

ruibai wrote: post #26

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

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

#44

Post by chadgeidel » Thu, Jul 21, 2011, 10:10 PM UTC

dpark wrote: post #39

daimyoyo wrote: post #34

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.)

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…

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

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

#45

Post by randallsquared » Thu, Jul 21, 2011, 10:18 PM UTC

bauchidgw wrote: post #15

InclinedPlane wrote: post #10

Earlier quoted context omitted.

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.

We tend to assume that the real world is not switched left-right, because when we're in that position, we don't feel switched left-right.

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

#46

Post by omaranto » Thu, Jul 21, 2011, 10:23 PM UTC

kenjackson wrote: post #29

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…

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?!

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.

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

#47

Post by omaranto » Thu, Jul 21, 2011, 10:25 PM UTC

khafra wrote: post #21

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.

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.

You could try with a human head turned sideways.

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

#48

Post by bauchidgw » Thu, Jul 21, 2011, 10:27 PM UTC

brown9-2 wrote: post #37

bauchidgw wrote: post #11

Earlier quoted context omitted.

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)

If there was a person standing in front of you instead of a mirror reflection, he would be blinking his right eye when you blinked your left eye. That's where the confusion comes from - you are thinking of the mirror man's blinking eye as "left" from your perspective whereas everyone else (and the original question) refers to it as the right eye from the perspective of the man in the mirror.

ok, now i get it. still it bothers me why anyone would think that a mirror switches left/right when everybody who looks into a mirror can clearly see that this is not the case. i never thought of myself as an empericist, but it seems i am.

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

#49

Post by omaranto » Thu, Jul 21, 2011, 10:30 PM UTC

Jtsummers wrote: post #20

bauchidgw wrote: post #11

Earlier quoted context omitted.

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 ca…

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.

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

#50

Post by omaranto » Thu, Jul 21, 2011, 10:32 PM UTC

adyus wrote: post #13

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 pow…

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.