Why Mirrors Flip Left and Right, But Not Up and Down

Stand in front of a bathroom mirror, raise your right hand, and your reflection seems to raise its left. But when you nod, your reflection nods right back. No confusion between up and down. It's as if the mirror is only concerned with left and right, completely ignoring vertical directions. Why does this happen?

People often say mirrors "reverse left and right," but that's not quite accurate. A mirror simply flips images front to back, along the axis that's perpendicular to its surface. Hold up a sign that says "SNAP" to the mirror and you'll see it read as "PANS", mirrored, but still right side up.

Picture yourself facing north, looking into a mirror attached to a wall. Your head is still at the top, your feet are on the bottom, left and right stay the same, but your image appears to be facing south, looking back at you. The swap is between "in" and "out" rather than left and right. We only get tripped up on left and right because our brains imagine stepping into the mirror and turning to face ourselves, as if switching places with the reflection. In reality, when you turn around, left and right reverse, but up and down do not.

This quirk is why the word "AMBULANCE" appears reversed on the front of emergency vehicles. Mirrors reflect the path of light straight back, so your brain interprets everything in its usual vertical orientation. Only the depth, the direction in and out, gets flipped. Up remains up, gravity doesn't waver.

So maybe it's better to think of a mirror not as reversing left and right, but as giving you a glimpse of what things look like if you could pass through and turn to look back at the world. The flip is always inside-out, never top-to-bottom.