Science Questions

Why do mirrors reverse but not invert things

Wed, 4th Sep 2013

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Question

Mark Faesen asked:

If one looks in the mirror, the left and right side is reversed, so we see a "mirror image". However, the top and bottom is not. Why do we see left and right swapped around but not top and bottom?

 

Regards

Mark

Answer

Dominic - Well, in fact, its a bit of a myth that mirrors reverse things because Car wing mirrorif you look in the mirror, you wave your left hand, you look in the mirror, the waving hand its actually on the left. Its exactly the right side it should be. But if, lets say, I'm looking at Ginny and if I was sitting behind Ginny in a car and we stuck out our left hand, our right hand, wed agree as to which side was which. But of course, to face me, Ginny is sitting on a swivel chair. Shes turning around to see me. She has inverted her sense of left and right when shes turned around to look at me. Another that Ginny could turn around to face me will be to stand on her head. She is now trying to do that in the studio. I'm not sure that's going to end well, but of course, rather than inverting her sense of left and right, shes then inverted her sense of up and down. So, the inverse is in fact, when she turns around to look at me, rather than when I look in the mirror.

Chris - And so, the answer is

Dominic - The answer is, that nothing is actually inverted when you look in the mirror, but things are inverted when we face each other in the studio.

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It doesn't swap anything. Put yourself in front of it and then blink your right eye, you will find that the (reflected) light goes directly, from your eye, to the eye blinking in the mirror, and all in a straight line. So how would the mirror swap? What you are thinking of is rotations, not swapping. And you're not the only one :) One might be able to argue that as most of us do seem to think in this manner, we're all instinctive geometric mathematicians. because what you are doing is comparing your reflection to what would happen if it was you there instead, 'rotated for real'.

You having rotated yourself, looking in the opposite direction.

It's less confusing if you think of it that way. In fact it is you, presuming that the mirror must fit your idea of what really should happen if you were to turn around, not any strange physics about it. And that's also what make us 'instinctive geometric mathematicians' in my eyes :) Although, applying logic I'm sure you agree on that there's no swapping happening.

You can also take a look at The Mirror Question for some more cool discussions about rotations. yor_on, Sun, 4th Aug 2013

It does swap things around, but it's the front and back that get switched round rather than left and right. The reflection of your left side is still on the left just as the reflection of your head is still at the top, but if your face is facing east in the real world, in the reflection it is facing west - that is what has been reversed. David Cooper, Sun, 4th Aug 2013

Assuming you was the one looking out from the mirror then David? We have this ability to imagine ourselves into something, like 'steering by ones pants' placing ourselves where we're not. Although to me that mirror image doesn't look at anything, it's a reflection, a depiction of the 'light paths' taken from you and your surroundings to the mirror, then reflected back to your eye. You can create rotations of something, as in the link above, but that's not the same, to me, as a mirror image would be. Because if you did a real rotation, then moving your left hand indeed should be the right, looking at 'yourself' in some weird space. yor_on, Sun, 4th Aug 2013

Its because your two eyes are side by side and not one above the other, if you want to see things inverted vertically lie on your side and look in the mirror syhprum, Sun, 4th Aug 2013

There is no rotation. It's a simple front-back reversal which is being mistaken for a left-right reversal. You don't make the mistake of thinking it's a top-bottom reversal because you aren't symmetrical in that direction (unless you lie on your side in front of the mirror, at which point you will mistake the front-back reversal for a top-bottom one). David Cooper, Mon, 5th Aug 2013

If it is a flat mirror, the paths of light will simply reflect without crossing paths. For true inversion/reversal you would need a parbolic mirror (like in a telescope) which would invert both vertical and horizontal.

I guess you could play around and parabolise (if that's not a word, it should be!) the mirror horizontally and not vertically, then you would see yourself as others see you and it would be very disconcerting. bizerl, Tue, 6th Aug 2013

Sort of fun isn't it? On one plane it is what most expect to be 'correct'. On the other tentacle you would have this nagging feeling of being conned somehow. yor_on, Tue, 6th Aug 2013

If you say left and right are switched, I think it is fair to say top and bottom are switched as well. Left and right are only switched from what you would expect by rotating 180 degrees about the vertical axis (ie. if you turned around). Similarly, top and bottom are reversed from what we would expect by rotating 180 degrees  about the horizontal axis (ie. if you turn upsidedown). chiralSPO, Sun, 11th Aug 2013

well for instance if you hold a card that says A when you hold it in front of the mirror its inverted so why dont we see ourselves inverted?????????????? guest, Sat, 19th Apr 2014

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