“Mirrors reverse left and right, but not up and down.”

It's a common experience to look into a mirror and see your reflection raise its "opposite" hand, leading many to conclude that mirrors mysteriously reverse left and right, but not up and down. This widespread belief stems from our everyday interaction with reflections, where we instinctively compare our physical selves to the image before us, noticing the apparent horizontal flip.

The scientific reality, however, reveals a more straightforward optical phenomenon. Mirrors do not selectively reverse along a horizontal axis. Instead, they reverse what's in front with what's behind, along the axis perpendicular to the mirror's surface. If you were to point directly at a mirror, your reflection would appear to point back at you, effectively reversing the depth dimension. This front-to-back inversion is the true action of a mirror.

Our brains are the real culprits behind the left-right swapping illusion. We are accustomed to seeing objects from various perspectives, and when we look at our reflection, our minds try to make sense of the inverted image. Because we have a clear sense of our own left and right, but not an inherent "front" or "back" orientation in a two-dimensional reflection, our brain interprets the depth reversal as a horizontal one. It's a cognitive trick, where our perception of our own body orientation leads us to believe the mirror has performed a selective flip.