A mirror flips left and right, but not up and down
physics.stackexchange.com
physics.stackexchange.com
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.
Yes... but not just intellectual.
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
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.
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.
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.
In the same way a teenage girl gains a following after making videos of how to apply makeup, I could easily see Feynman becoming a similar YouTube phenomenon had he lived now and released videos like this on a regular basis.
Always fun to imagine people from another era who you know would have been MASSIVE on social media (another one: Robert Townsend -> Twitter).
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.)
The effect of left/right "flipping" is because we are left/right symmetrical and so we can imagine ourselves being the person in the mirror by turning that way. For up-down flipping to happen we have to be top-bottom symmetrical to even begin to think about it, although it might not be that simple since gravity continues to differntiate those directions.
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.
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.
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.
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.
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.
"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.
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."
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)
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!
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.
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.
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?
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.
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.
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.
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.
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.
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.
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?
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.)
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.
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.
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.
Flipping the "switch" on the mirror simple changes the angle of the mirror by the exact amount needed to reflect the light from your rear window into your eye off either the silver surface or the glass surface.
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)
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.
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.