Why mirrors flip left-to-right but not up-and-down

If you’ve ever looked in a mirror, you’ve likely noticed that everything you see is flipped. When you raise your left hand, your reflection raises their right hand. When you wink with your right eye, your reflection’s left eye winks back.

And if you write a message and hold it up, you’ll see your reflection hold up the identical sign, but everything appears backward, even the individual letters themselves. It appears that everything you see reflected in the mirror has their left-and-right reversed. But, for some reason, up-and-down don’t appear to be reversed.

Your mirror reflection still has their feet on the ground, their ceiling up above, and while the letters on your mirror image’s writings are flipped left-to-right, they all remain right side up. It’s not just humans who experience this: whether you’re a sea star, an insect, a jellyfish, a parakeet or a cat — whether in space or on Earth or anywhere else in the Universe — you’ll still see the same thing. When you look into a mirror that’s mounted on any wall, they all appear to reverse left-and-right, but not up-and-down.

This isn’t a deficiency in your mirror at all; it’s a consequence of how reflections work at a fundamental level. When you reflect text in a mirror, both each letter and the letter order appear reversed. If the text can be read both ways, in reality and in the mirror, it’s known as a mirror ambigram.

The text here reads “optical” on the left, but “let it go” on the right. The first thing you have to recognize is there’s nothing special about our environment here. There’s nothing remarkable, as far as mirrors and reflections go, about: We could turn gravity on or off; we could rotate ourselves by any angle, even 45°, 90°, or 180° about any axis; we could give ourselves additional eyes or senses; we could rearrange the objects surrounding us in any configuration we like.

Still, despite any of these modifications, we’d still see that up remained up, down remained down, and that everything in the mirror would appear as though left and right were switched. One of the best examples to illustrate this is to consider a spinning ball in the mirror, and to consider it from two perspectives: one of a ball spinning about its vertical axis, like a basketball on an adept athlete’s finger, and one of a ball spinning about its horizontal axis instead. From the perspective of Harlem Globetrotter TNT Lister, the balanced, spinning basketball appears to rotate from her left toward her right: the same as for the child on whose finger the ball is being transferred to, or counterclockwise.