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

#31

Post by ars » Thu, Jul 21, 2011, 9:07 PM UTC

hammock wrote: post #28

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…

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…

Rotating the object will not get you the same view as in the mirror. You would need to flip it inside out.

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

#32

Post by [deleted] » Thu, Jul 21, 2011, 9:14 PM UTC

hammock wrote: post #28

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…

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…

[deleted]

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

#33

Post by ldite » Thu, Jul 21, 2011, 9:16 PM UTC

hammock wrote: post #28

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…

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 don't know about you, but if I turn a picture-postcard around its vertical axis, left-to-right, all I see is the blank back of the picture. (Likewise if I rotate it around it's horizontal axis.)

What you're actually saying is if I put the picture on transparent acetate (or whatever) and rotate it, then look at it through its back, I see the same as I would in the mirror. Which is essentially the same as reversing the depth of the object (when you think about it.)

And there's nothing privileged about the Left-to-right rotation. When you face the mirror and hold the picture up (facing the mirror, away from you), you chose to rotate it around its vertical axis to face the mirror (thus swapping left and right). If you chose to rotate it around its vertical axis, top-to-bottom, to face the mirror and away from you then you get the other situation.

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

#34

Post by daimyoyo » Thu, Jul 21, 2011, 9:16 PM UTC

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.)

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

#35

Post by dpark » Thu, Jul 21, 2011, 9:18 PM UTC

hammock wrote: post #28

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…

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…

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.

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

#36

Post by colanderman » Thu, Jul 21, 2011, 9:18 PM UTC

hammock wrote: post #28

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…

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…

It's psychological. You could instead rotate the object head-over-heels to "simulate" reflection, and the object would then appear upside-down, but have left and right in the same position as on a mirror. However things don't normally move head-over-heels but rather by rotating left/right (thanks to gravity) so psychologically, we assume the object must have rotated left/right to get on the other side of the mirror, thus appearing reversed in that direction.

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

#37

Post by brown9-2 » Thu, Jul 21, 2011, 9:26 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)

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.

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

#38

Post by anigbrowl » Thu, Jul 21, 2011, 9:31 PM UTC

hammock wrote: post #28

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…

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…

It doesn't flip left to right. It flips front to back. Hold something in front of a mirror; you > thing > mirror > thing > you.

It reorients things 'on the vertical plane' parallel to your body because your eyes are perpendicular to your vertical orientation and your brain is used to processing things that way. Put a mirror on the floor or the ceiling and mess about with some geometric solids for a while, you'll start to think differently about it. Alternatively, get some dark safety goggles, a laser, and a bunch of small mirrors on adjustable fixtures.

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

#39

Post by dpark » Thu, Jul 21, 2011, 9:31 PM UTC

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 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.

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

#40

Post by losvedir » Thu, Jul 21, 2011, 9:34 PM UTC

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.)