calvin mccarter

Observant people often wonder why mirrors appear to flip left and right, but not up and down. If you search for scientific explanations of this, you’ll read that mirrors don’t flip left and right, but rather forward and reverse. This is correct as far is it goes, but it’s not fully satisfactory. Why do we instinctively feel like mirrors flip left and right? Why do we not instinctively feel like mirrors flip up and down?

The answer is that left and right are relative directions, while up and down are not. If you turn yourself upside down, you still consider “up” to be the direction opposite gravity: you heed your inner ear, not your eyes. There is no non-visual reference for left or right, so as you turn in either direction, you redefine their meanings. If a mirror were to “flip up and down,” it would have to actually depict your head floating above the ground, opposing the absolute direction of earth’s gravitational field.

Not only do people define left and right in relative terms, unlike up and down, we also do something strange with mirrors: we anthropomorphize the image. What we mean when we say that a mirror flips left and right, is that if we cloned ourself and then walked around 180 degrees to face ourselves like the image in the mirror, we would appear left-right-flipped compared to the image. It is our imagination of the image as being our spun-around-and-facing-us clone that caused the mirror to defy our expections.

The full answer to the mirror-flipping question involves both physics (how light reflection works) and psychology (how we think about directions and mirrors).