When you look in a mirror, it feels like your left becomes right and your right becomes left. But strangely, your head is still at the top and your feet are still at the bottom. So what’s going on? Is the mirror doing something special to left and right but not to up and down?
The short answer: the mirror doesn’t actually flip left and right at all. It flips front and back.
Imagine you’re facing north, looking into a mirror on the wall. In the real world, your nose points north. In the mirror image, your nose appears to point south, straight back at you. The mirror takes every point in front of it and sends it the same distance behind the mirror’s surface. That’s a front-to-back reversal.
So why do we talk about left and right being reversed?
Because of how we think about turning our bodies.
Picture yourself holding up your right hand. In the mirror, the hand that appears to move is on the opposite side from your point of view. But that “other” hand belongs to the image person, who is facing you. If you could magically step into the mirror, you’d have to turn around to face the same way as your reflection. When you turn around in real life, you rotate around your vertical (up-down) axis. That rotation swaps your left and right sides, but it doesn’t swap top and bottom.
In other words, your brain imagines, “If I were that person in the mirror, turned to face my way, my left and right would be swapped.” So it feels like the mirror flipped left and right, when really it just flipped front and back, and your mental rotation did the left-right swap.
What about up and down? We don’t normally flip ourselves upside down to “match” the reflection, so we don’t mentally swap top and bottom. Your head stays up, your feet stay down, and it matches what you see.
So the mirror isn’t picking on left and right. It’s simply reversing depth. The rest is a clever trick played by your own sense of direction.