It is a common misconception that mirrors reverse sides. They don't, and it's easy to prove. Tie a ribbon on your left hand and look in the mirror. The ribbon stays on your left where your heart beats (Fig 1). Our body has bilateral symmetry, meaning our left and right halves look similar. We accentuate it further by favouring symmetrical hairdos, clothes, and shoes. So, it takes a particular item on one side of our body to break this symmetry and make us realise what is left and right in the mirror.
Fig 1 The ribbon on the woman's hand stays left in the mirror.
If you're still not sure, I don't blame you. Mirrors are notorious for playing tricks on our minds. I believe the best proof is the proof by contradiction. So, I'll show you what you'd see in the mirror if it had reversed sides. In Fig 2, a scarf and pom-pom on the man's left side moved to the right in the mirror, and the glove on his right side moved to the left. Is that what you see? Certainly not.
Fig 2 Hypothetical scenario of the mirror reversing sides.
There is another widespread misconception that mirrors reverse text. They don't; you do. Take a piece of paper with text and mark the sides with L and R. When you read the text, your left hand holds the L side, and your right hand the R side. If the paper were transparent, you could read the text in the mirror from this position. Otherwise, you must turn it over to face the mirror. And then your left hand will hold the R side and your right hand the L side. You swapped the sides and not the mirror. And that is why the text's reflection became unreadable.
The only direction mirrors reverse is front to back
So, we established that mirrors don't swap sides. They also don't reverse top to bottom, as we don't see our reflections turned upside down. The only direction mirrors reverse is front to back. In Fig 3, a woman stands in front of the mirror. Both side arrows, red and orange, point to her left shoulder. Both verticle arrows point to her head. But the arrows she and her reflection hold in their hands point in opposite directions. If we extend the original direction with the dotted line, we'll see that the mirror has also reversed the order of her body. While her left foot and ponytail point to Back in the real space, their orientation has switched to Front in the virtual mirror space. We must swap Front to Back in the mirror to restore the original order. We can see what's behind us in the mirror because it reverses front to back.
Fig 3 Mirrors reverse only one direction, front to back.
So, what would happen if mirrors didn't reverse front to back? We'd have that horror film scenario where characters see their backs instead of faces. In Fig 4, the mirror has not changed the original order, and the arrows in a woman's hands point now in the same direction. This looks so alien to us that raises an interesting question of what our lives would be like if that were the case. Exploring alternative reality can help us better understand our own. So why not indulge in a flight of fantasy?
Fig 4 Hypothetical scenario of the mirror not reversing front to back.
If that were the case, we would never see our faces and know how we appear to other people. With our best efforts, we would only catch a glimpse of our semi-profile from behind. Unless, perhaps, we had the wide-positioned eyes of a chameleon who can see behind without turning its neck. In Fig 5, the mirror shows a woman's face but the woman can't see it. If she turns towards the mirror, her reflection will turn away from her. Even a second mirror would not help. In our world, the second mirror in front of the first reverses the first mirror's reflection. But without this ability to reverse, the second mirror would still show the woman what shows the first. That is her back.
Fig 5 Woman is trying to look at her face in the hypothetical scenario of the mirror not reversing front to back.
You can try this trick with the second mirror to better understand this alternative scenario. We use the second mirror to see our backs, but we may not realise that it also reverses the scenery and shows what's in front of us instead of behind us. That's what the mirrors in the alternative reality would show: our own backs and the scenery in front of us. This will turn rear-view mirrors into front-view mirrors, which would render them useless and cars too dangerous to drive.
Lucky for us, this scenario can't happen in our world because it defies our laws of physics. We see reflections because the light that hits a mirror reflects off it, reversing its original direction. Just like a ball that hits a wall bounces off it, changing its direction from towards the wall to away from it. The light carries information about the objects behind us because that's where it came from. Snell's law states that the angle of reflection equals the angle of incidence. So a mirror doesn't change the information the light carries, it simply reverses it from towards the mirror to away from it. Thus making reflections look as real as the view behind us.
Note that all directions are relative to mirrors and not to observers. So, a reversion from front to back should be treated as from towards the mirror to away from it. In Fig 6, the mirror is now placed underneath a woman. It may look to us like her body has reversed top to bottom. But applying directions correctly, we see that it has reversed from towards the mirror to away. In Fig 7 we have a similar case. It may look like the text has reversed left to right, turning 3.14 into PI.E. However, the writing has again reversed from towards the mirror to away.
Why do we think that mirrors reverse sides?
We think that our reflections reverse sides because we have to rotate ourselves 180° about our axis of symmetry to assume the position of our doppelganger in the mirror. What we really should do is to turn ourselves out front to back. But we have no experience doing this in real life, and our imagination can't stretch beyond our life experiences. So we settle for the second best.
Confusion becomes particularly prominent when we try to manoeuvre a car using rear-view mirrors. The mirrors show the position of objects behind us relative to our doppelganger. So, to make a successful manoeuvre, we must first rotate ourselves 180° to take the place of our image in the mirror and then reverse sides, restoring the right order. Experienced drivers have trained their brains and perform this well. Newbies, however, always have problems It is by no means an easy task.
If the human brain can cope with one mirror, two mirrors will get it totally confused. Just try to trim your back hair using a second mirror! Your scissors will stubbornly move left when you want them to move right and vice versa. Until you decide enough is enough and declare defeat.
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