As your form began to pancake, the front half of your body that is, all parts of your body situated behind the tip of your nose, but still in front of the back half of your body , the back half of your body and the tip of your nose all came to reside within the same plane i. This new, inverted you is symmetrical to you, but your two bodies cannot be superimposed.
In chemistry, such entities are said to be "chiral. Here's another way to think of it, widely popularized by physicist Richard Feynman see the interview response featured here. Stand in front of a mirror, and note which direction you're facing. For the sake of this thought experiment, let's assume you're facing North. Point due East with your right hand, and your reflection points East as well. Point due west with your left hand, and your reflection gestures in the same direction.
That's because these directions both lie along a plane parallel with the mirror. Similarly, point up or down and your reflection will follow suit, motioning in the same direction. And we typically set forward to be the direction in which we're facing.
We can make other choices of course, and we can easily define such directions for a machine, so the general idea is not really anthropomorphic. Ask two people facing each other to point up: they'll both point in the same direction. Now ask them to point right. They'll point in opposite directions. They have each chosen one additional direction, say front, before they could specify the direction of right. And because those two people have chosen opposite directions for their front, this has also altered their definition of right.
It's remarkable that the laws of physics also have a notion of this "handedness", including a particular choice that Nature makes in one field of particle physics. But, of course, as children we don't learn our concept of handedness by studying particle physics.
Suppose I have a clear notion of which is my right hand. I notice that if I open my right hand and point my fingers forward, then close the fingers in such a way that they rotate to the "up" direction, then my thumb points right. Notice that if any one of forward or up is reversed, the direction of right will be reversed too.
This is an example of applying the "right-hand" rule to sort directions out. The ordered triplet of forward—up—right becomes a "right-handed" set of axes. Mathematicians use this idea to distinguish between sets of axes in three dimensions: the only options for such mutually perpendicular axes are left handed and right handed. No matter how you rotate them in space, these two sets of axes can never properly "become" each other, just as our hands can never become each other.
So when I look into a mirror, what do I see? I see an image of myself that is actually reversed forwards-backwards. That's a strange sort of reversal, and one that's not immediately obvious when we see our image. If I have a freckle on my left cheek, what do I see in the mirror? Just as that freckle is on the same side of my body as my heart, so too the man in the mirror has the freckle on the same side of his body as his heart.
So he must have the freckle on his left cheek too—and the mirror has not reversed left-right! Mirrors do not flip left to right. They flip front to back. Mirrors reflect light rays such that the angle of reflection equals the angle of incidence. For a three-dimensional object standing in front of the mirror, there is an image of the object created behind the mirror with the right side still on the right, the left side still on the left, the top side still on the top, the bottom side still on the bottom, but the front side is flipped to the back and the back side is flipped to the front.
This can be easily seen by tracing the rays from significant points on the object and seeing where they end up in the mirror image.
The drawing on the right shows the result, which confirms that mirrors only flip front to back. But ambulances indeed seem to have the word "Ambulance" painted in a manner that is flipped from left to right so that drivers can read the word the correct way when looking in their rear-view mirror.
Doesn't this show that mirrors flip left to right? Actually, the word "Ambulance" is not flipped left to right, or any way at all. It seems to be flipped left to right when you turn your head to look at the ambulance.
But that is a result of you turning your head, and not of the mirror. If you hold up a word to a mirror so that you can see both the word and its image in the mirror at the same time, you will see that they both read the same direction and there is therefore no right-left flipping. This effect is captured in the image below. If you now hold the word behind you without turning the word, and then turn your head to look directly at the word, it now does appear flipped.
That question would be: Why do mirrors reverse up and down? There is something that mirrors do flip. They flip front and back. Just imagine that you were Obi-Wan and you walked forward a bit to where the image would be. Your right hand would be on the right side still. However, your back would be facing towards where the mirror was, not your front. So, mirrors flip front and back. I guess I should state the full question: Why do mirrors reverse left and right, but they don't reverse up and down?
Rhett Allain is an associate professor of physics at Southeastern Louisiana University.
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