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If you write a word on a piece of paper and present it to a mirror, it looks backwards in the reflection, so we think a mirror flips left and right. We see our head up top and feet down below, so we think a mirror doesn’t flip up and down.
But if you rotate the paper 90º so that the first letter is closer to your head and the last letter is closer to your feet, you still need to read it “backwards” in the mirror. And if you look into a mirror while lying on the ground on your right side, the sky is still on the left in the reflection, and the ground on the right. The sky and ground would be reversed, though, if you took a photo in that position and applied a left-to-right filter, so we know that’s not what mirrors do.
Mirrors flip depth along the plane of the mirror. If you face north and plop a big ol’ mirror down in front of you, taking a step west causes your reflection to take a step west, and the same goes for east. If you raise your hand upward, so does your reflection. But if you take a step north, your reflection takes a step south.
For thin objects, flipping depth is like flipping back and front. If you write something on a piece of paper with a thick marker so that you can see it from both sides, and hold it up to a mirror, what you’d see looking at the actual paper would match what you see in the mirror—either “backward” text or “forward” text. The reflection shows the opposite side of the paper than what you see when you look at it directly, but because it’s flipped front-to-back, the real paper and the image of it appear to match.
But it’s confusing to think about what that means when you’re looking at a person. People aren’t transparent, so when you look at an image of one where depth is inverted, you don’t see their backside. When you touch the mirror, it’s hard to see the reflection as anything but a second person whose right hand is touching your left hand. Oh well, it’s just how we’re wired.
As I was wrapping this up, I asked my wife why mirrors flip left and right but not up and down. She said it’s because they would be too confusing to use otherwise. If there’s anything you take away from this post, let it be the value of having a second perspective.