Dear Cecil,
Why does an image in a mirror appear to reflect left and right and not top and bottom?
- John Hughes, Nottm, England.
Every part of a mirror reflects what’s in front of it. Try looking at yourself while lying down.
What’s on your left is on your left i the mirror. What’s on your right is on your right in the mirror. What’s above you is above you in the mirror. What’s below you is below you in the mirror. It doesn’t do anything different left/right than it does top/bottom.
In fact, try to look at yourself from behind yourself: imagine stepping one step backwards, out of your body. Now you see yourself from behind. But now imagine that every part of your body except the front layer is perfectly translucent: then, the light that will reach you will come from every point of your front surface. The image that you see is the image the mirror reflects back, and in which, in fact, everything to the left/right of you as you stand in front of the mirror is still to the left/right of your mirrored image, as engineer_comp_geek pointed out. Nothing is reversed; in fact, the image we expect to see is a rotated one, which you’d get from stepping out of your body forward, and then turning around.
The late Martin Gardner pointed out what is going on in a mirror. Top and bottom, and left and right all stay where they are in a reflection. It is front and back that are reversed.
This has the effect of appearing to make left and right reverse - but they are actually still where they always have been.
I think this is a question of the impression we get when we think about this, as there’s no physics or geometry subtle enough here to be interesting.
If you stand in front of a mirror and have a friend stand almost behind the mirror (where you can still see them) facing you, and then you do an experiment by having your friend move around to your side while you watch them and watch their image in the mirror, you will see left and right change in your view of your friend. But, notice, it’s really because your friend turns around in the process, and does so about a vertical axis. If your friend could levitate and did this exercise in a vertical plane about a horizontal axis, suddenly it would be up and down that the mirror mystically reverses.
You are talking about a mirror that is mounted vertically. Lay it flat on the floor, and you’ll find that it reverses the up and down, but not the left and right.
The mirror isn’t doing anything.
Hold some paper, with writing on the far side, up to the light… So you see the writing is backward, right ? Well the mirror isn’t going to change it… You use a mirror, and look at it, you see that backward writing.
How can you see writing in a mirror UNLESS you flip it around to be facing away from you ? Its that flip which makes it look backward. Or if you flip it vertically, it looks upside down, BUT NOT BACKWARD… See ? Its the movement of putting the writing in view of the mirror that makes it hard to read the writing !
Thats what the mirror is sending back to you! The mirror isn’t flipping anything, you did it when you put the writing in that direction !
If you do flip the paper horizontally, THEN you can read the writing, with no mirror.
Oh actually if you hold a piece of paper in front of you, with writing on the close side, and the writing is normal… then it is possible to hold a mirror and use the mirror to see the same paper, and then the writing is backward… or something. If the mirror is above or below the writing, then the writing is upside down… there’s that funny angle inbetween which is something inbetween upsidedown and backward…
Try it with writing on a piece of transparent film (or just thin paper and ink that soaks through) and it’s even easier to see what’s going on - but you’re right -it’s not the mirror that reverses the text, it’s you, when you turn it around to face the mirror.
If you want to see your reflection without right and left being interchanged, use a pair of mirrors that meet at a right angle, with the line of intersection vertical. (When you have three mirrors that meet at mutual right angles, it’s called a “corner cube” or a “cube corner”) They sometimes use this for clothes fitting, although a set of three m,irrors, with a central mirror flanked by two others at less than right angles is more common for that purpose.
The shortest explanation I’ve heard is this:
Mirrors don’t reverse Left-Right and they don’t reverse Up-Down. They reverse Front-Back.
When you stand in front of a mirror and you point up, or down, or right, or left, your reflection points in the exact same direction you do. But if you point forward or backward, your reflection points in the opposite direction from where you are pointing.
If you still aren’t convinced that mirrors DON’T reverse Left-Right, try this experiment: Get a piece of ordinary typing paper and a heavy black marker whose ink will bleed through to the other side of the paper. Write your name on the paper. Now hold it up in front of you so you can read your name. Now stand in front of a mirror. Hold the paper exactly the same as you did a moment ago, so you are looking at the front side and the back side is towards the mirror. Look in the mirror at the bleed-through. Is your name reversed? No. Now flip the paper over so the front side is facing the mirror and look in the mirror. Is your name reverse Left-Right or is it reversed Up-Down? The answer can be either one, depending on how you flipped over the paper! Did you flip the paper Left-Right or did you flip it Up-Down? The mirror didn’t reverse anything. You reversed it yourself when you flipped the paper over.
This is actually a frame of reference question. We get confused because (a) we are dealing with objects that are overly familiar and (b) we are not consistent with the reference frames that we commonly use.
Concerning (a), we are very used to having mirrors mounted vertically on a wall and looking perpendicularly at them. Put them in another orientation and we really don’t deal with it well. Try doing a simple manipulative task where the only image you have of what you are doing is through a mirror positioned at an unfamiliar angle. Give someone a mirror and ask them to position it so that they appear upside down and for most people it will take them a while to think of mounting it on the ceiling. Tell them to mount it so that if a person is standing their reflection will be lying down and you will often see all kinds of crazy rotating in the plane of the mirror before it gets angled 45° to the plane of the wall.
Concerning (b)
Left and right are defined with respect to the body we are describing. You have a left hand and a right hand which are defined relative to you. Your image has a left and a right defined relative to the image.
Up and down reference frames are different. We tend to use only one reference frame – ourselves. Thus if we see an image in an orientation that we would call upside down we do not really consider that the image still has the ceiling “above” their head.
Front and back are similarly defined by our own world. So if I am approaching a mirror and moving northward my reflection is travelling south. It seems to me that in the case of wall mounted mirrors we think less often of forwards and backwards and more commonly think of closer to and further from. In this case the physical mirror becomes the frame of reference.
The truth is we understand how mirrors work. But our language has developed for the situations that we commonly encounter them. Thus we lack straightforward language to describe what is going on.
Cal, I’m disappointed in you. What you just described is a mirror that does interchange left and right, unlike a normal mirror which does not.
As to why people mistakenly think that an ordinary mirror flips left and right, it’s largely because humans are (almost) bilaterally symmetric left-to-right. The image you see in a mirror looks like an inversion of a human, but because humans are almost symmetric, it also looks almost like an actual human. And if it were an actual human, the arm that’s on your left would be on that person’s right. But it’s still on your left.
You misunderstand – I mean that the figure in the mirror raises its right hand when you raise your right hand, unlike a normal mirror that raises its left hand when you raise your right. So the mirror does NOT interchange left and right, unlike an ordinary mirror, which does.
It’s your usage that’s backwards.
The very simple answer is this. When you turn around to look in a mirror, you rotate your body around a vertical axis, which reversed right and left.
Stand with your back to a mirror, right hand touching right hand and left hand touching left hand. See, they are not reversed. Now walk away from the mirror, and turn around and look at it. Did you reverse your right and left, or your top and bottom?
When I raise the hand on my right, my image also raises the hand on my right. No reversal. If you start talking about “the image’s right hand”, you’re imagining the image as being a person who walked back there and turned around. But it’s the imagined turning around that reverses left and right, not the mirror.
Correct. If I’m facing you and we both raise the hand on my right side, you’ve actually raised your [left hand.
Since you’re the stand-in for my image, my image raised its left hand.
It’s not my fault you’re my-side-of-the-mirror-centric.
That depends on how I stand in for your image. If I stand with my face lined up with the image’s face, then I’m raising my left hand. If I stand with my heart lined up with the image’s heart, then I’m raising my right hand.
As Og is my witness, I know that the Master answered this in one of his books (the first one I think). It doesn’t seem to show up in a search for “mirror”. I’m blaming the Illuminati.