Interview Questions And Answers 
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Imagine you are standing in front of a mirror, facing it. Raise your left hand. Raise your right hand. Look at your reflection. When you raise your left hand your reflection raises what appears to be his right hand. But when you tilt your head up, your reflection does too, and does not appear to tilt his/her head down. Why is it that the mirror appears to reverse left and right, but not up and down?
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Left and right depend on the direction you face. Up never changes when you change direction.
I think the reason of this doesn’t directly relate to the mirror. It’s mainly because man himself is “left-right” symetric but not “up-down” symetric.
It may be because, our eyes are placed left to right when facing the mirror. So the reflection from mirror crosses like a X in front of us in horizontal direction. As far as vertical reflection goes, the eyes are in same line. So the vertical reflection will be just like a horizontal line without reversals. That’s why the image appears reversed horizontally and not vertically.
humans are left right symmetric and not up down thatsthe reason
As someone mentioned before: this happens just because the human perception of bilateral symmetry in their bodies.
If you wear a red (have to be red) that says “left” (translate to your own language) on your left hand and raise it in front of the mirror you will not be confused. This happens because you destroyed the symmetry…not because something is written on the glove.
You have all mentioned another symptom and not the cause - the reason is due to the relative position of our eyes.
If our eyes were located one above the other we would still be able to determine distance but we would then have the opposite scenario in the mirror!
Well, I don’t think, its because humans are left-right symmetric and not top-down!!!
Just stand in front of a concave mirror (or might be the convex - I don’t remember, which exactly. Been some time, since I’ve read this physics) beyond the focal length and try the same excercise - one would obviously find the top-down symmetry also coming into play.
I think, the reason is that flat-mirrors’ focal length is infinity and we always stand within its focal lenght and don’t see the image inverted.
Correct me, if I’m wrong!
When you tilt your head up, it is actually tilting back. Since this motion is perpendicular to the mirror, you won’t see the reversion like the hands. However, your head in the mirror is tilting backing in the opposite direction. This has nothing to do with the left-right symmetry or the human eyes.
This is not a Physics issue… this is a matter of perception.
When you stand in front of the mirror and raise YOUR LEFT HAND, the reflection is of YOUR LEFT HAND.
By referring to “what appears to be his right hand” the question has essentially asked you to envision yourself rotated 180 degrees about a vertical axis, thus subtly introducing the apparent conflict between left and right.
Why it works: Since humans are relatively symmetric along a vertical axis you are able to envision YOUR LEFT HAND as a suitable RIGHT HAND for your reflection.
Humans are not generally symmetric along a horizontal axis (hey, I’ve never seen it, but that doesn’t mean some freak isn’t out there…), so it’s much harder to envision feet as a suitable head and vice versa.
My Goddness!!!
I would really get crazy if it doesn’t work this way.
Note of translator: Loco serķa que no fuera asi!!!!
Think about it!!
Naccio
This is all perception issue.
When you look at the mirror, and you stretch
your right hand you really looking up to the
right not to the left. So it is all in your
mind when you think you are looking at a
different person in front of you and you think he stretches his left hand.
And it is all for the reason that a human is
symetric with regards to the vertical line.
If a human was symetric with regards to a
horizontal line it will look up and down switched down and up respectively.
It is a perception issue, but the principle is that of optics.
Now the mirror, assuming a normal mirror.
By the property of light, light waves (which is a vector) after hitting the surface of the mirror travel back in the same direction (Although not 100% is reflected back.
Because We All Have only ONE HEAD!
We should have 2 of the rest (hands, legs etc).
Its totally a matter of perception. Try this.
* Bend to any of your side and lift an arm up and hang the other one down.
* Now try to visualize this not as left and right but one arm up and one arm down.
* Now move the arm thats up.
* The mirror image also shows the arm thats up as moving.
In this case you don’t find that problem because the way you are visualizing it.
The question itself causes the brain to think the reflection as a copy of itself rotated 180 degrees (as explained by SomeGuy).
The mirror is completly flat and does not distinguis between horizontal and vertical.
In this case the even the eyes are (nearly) vertical.
Even when we see that the upper arm is moving, if we start to think about the reflection as a person. Then is the time when the illusion takes effect.
This is not an answer, but a question to get the discussion on track. If the Left-Right vs Up-Down mirroring was an issue of perspective, why does a photo of the mirrored image held upside down still appear upside down. If it was a perception issue, wouldn;t the brain flip this as well?
Let me try.
If I move any object or any part of my body to left, What I see in the mirror is its moving to left and whatever I move towards right, in the mirror it moves towards right. That is the same case for top or bottom. Actually we are fooled in the question.
Mirror does not do the opposite of what you do. When you raise your hand it doesnt lower your hand it is just a rotation along the vertical axis. Instead if the hand is lowered in the reflection when you raise your hand which is a rotation along the horizontal axis then the head will also tilt down in the mirror when you actually tilt it up.
Say, an object contains a local xyz-coordinate where x-axis is facing towards the mirror and z is up. LEFT/RIGHT is an orientational quantity determined by the xz-plane of the local coordinate(x cross z is right, z cross x is left) while UP/DOWN is an absolute quantity in the local coordinate that just says positive z direction as opposed to negative z. Since mirror flips the x axis, the LEFT/RIGHT-ness also flips by its definition. Simply, LEFT/RIGHT is an orientational concept that is simply not the same geometrical concept as UP/DOWN.
To anyone who says two horizontal eyes has anything to do with it:
-close one eye
-look in a mirror
-think
We uses 3 axies (X, Y , Z) to define locations of our 3-D world. Once we have the direction of x- and y-axes specified, there are two possible ways to define the directions of z axis. The two possible coordinate systems which result are called ‘right-handed’ and ‘left-handed’ coordinate system. Therefore, reverse of the z orientation will convert a right-handed system to a left-handed system. Futher, the coordinate system with a reverted x or y axis can be rotated to have the equivalent effect of reverting the z axis.
If we put the mirror in XY plane, it reverts the Z orientation and this converts a right-handed system to a left-handed system or vice versa.
The definition of “right” and “left” is relative to the object orientation, which can be defined by a vector (x,y,z). The mirror changes the corrdinate system and hence the orientation vector, for instance, form (0,0,1) to (0,0,-1). That is why you left and right side is flipped in the mirror.
The definition of “up” and “down” is relative to the direction of the Y axis. When you put the mirrow in the XY plane, the directionof Y axis has is the same in the mirrowed corrdinate system. That is why you will not feel a flip of “up” and “down”.
Imagine that if you put the mirror under your foot (in XZ plane), the Y axis will have an opposite direction in the mirrored coordinate system. When you loop up, your mirror image will actually look down.
If we have a horizontal symmetric axis, say, imagine our leg and arm are in the same shape and we assign the arm as left and the leg as the right. When you strechout your arm, you would think that you strechout your leg in the mirror as if you lay down horizontally. This has nothing to do with “up” and “down”.
When you compare the original XY coordinate system of yours, to that of the mirror coordiate system.You can see that your + X axis coverted to -X axis and -X axis converted to + X axis.
But your +Y axis and -Y axis remains the same in both the coordiate systems.
Thats why the reversion occurs only along the horizontal direction (ie X -axis ) but not along the vertical direction(ie Y-axis)
see byte03 “Left and right depend on the direction you face. Up never changes when you change direction.”
Firstly, for those of you who think it’s to do with the plane of our eyes - do you really think one-eyed people don’t wonder the same question?
If I start standing up, then rotate 180 deg on the x-axis so I’m facing the other way, my left hand is where my right hand was, my right hand is now where my left was, my back is where my front was and my front is where my back was. I have flipped in 2 axes. My head is still where my head was and my feet still where they were.
(Note: “Axes” is the plural of axis and not the subject of an early Pink Floyd song.)
From standing, if I was able to do a cartwheel and stop halfway through it (180 deg on the z-axis), my head and feet would be exchanged, and my left and right also exchanged. My front and back are unchanged. Again - 2 axes flipped.
From standing, if I now do a handstand (180 deg on the y-axis), my head and feet are exchanged and my front and back are exchanged. My left and right are unchanged. Again - 2 axes flipped. Any 180 deg. rotation flips 2 axes.
When we see other people in orientations other than our own, we subconsciously apply this “2 axes” rule because that’s the way the physical world works. However mirrors are very tricky in that they transform the original into the reflection by reversing (not rotating) only one axis - the z-axis.
When I see myself reflected, I see a person facing me. My brain deals with that image by imagining it’s a version of me that has rotated 180 deg. However no such rotation has taken place, giving rise to the illusion that the reflection’s left hand is my right hand. Left and right are reversed and not up and down simply because of that “spurious” x-axis rotation applied by the brain.
I dont think so, it is something to do with symmetry. Take the case of somebody facing you, in that case his left is right for you and vice versa. Same is the case with the mirror where the next person is your image itself. So its because of reference with which I am judging left or r ight
We know, that image of any object projected by the lens on a screen flips left and l top and bottom. Your brain rotates back in vertical direction, and does not rotate in horizontal direction. Nobody can tell for sure why the brain had evolved this way.
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