Why does a mirror reverse left to right, but not top to bottom?
When this woman looks in the mirror while holding a brush in her right hand, it looks like her reflection is holding the brush in its left hand. It looks like left and right are reversed, but top and bottom are not. Why?
You may realize that the mirror is not really reversing left and right. The reflection of the brush in the woman’s right hand is a reflection of her right hand; it is not “left” anything. But still, why does her reflection appear to be holding the brush in its left hand?
The confusion comes from the use of the terms left and right, which are unnecessary and confusing in this situation. Consider the Aboriginal community of Pormpuraaw, where people speak Kuuk Thaayorre, a language that does not include relative spatial terms like left and right. Speakers of this language always use absolute cardinal directions to indicate relative positions of things, even on a small scale. Thus, if they happened to be facing north while holding something in their right hand, they would say it was in their “east” hand. If they turned around to face south, that hand would become their “west” hand. Residents of Pormpuraaw are much less likely than we are to become disoriented, and they would also not be confused by the question posed in this brain teaser. When looking in a mirror and holding something in their “east” hand, their reflection would also be holding the object in its east hand. Other confusing situations, such as “stage right” directions or “Do you mean my left or your left?” are never a problem for these speakers.
Can you make a mirror that does not reverse from left to right?
There is another aspect to this problem: when you look at yourself in the mirror, you are not seeing what others see. Because left and right appear to be reversed, and human beings are not perfectly symmetrical, your reflection looks different from the way you look in a photograph or when someone sees you face to face. If your right ear is a little higher than your left, or you part your hair on the right, your reflection will have it backwards. That is because what mirrors actually do is reverse from front to back. We may picture our reflection as if we walked into the mirror, then turned around 180 degrees and faced back at ourselves, but that is not what happens. The reflection we see is of a reversed person who does not really exist. So, can you make a mirror that shows what you really look like?
A non-reversing mirror is made simply by placing two mirrors at right angles to each other, and then looking into the corner. There will be a line down the middle, but otherwise you will see yourself as you really are, just as if you had walked into the mirror and then turned around 180 degrees. That is, your reflection’s right side will be your right side, and its left side will be your left side.
The images above show the difference between an ordinary mirror on the left and a non-reversing or “true” mirror on the right.