Why do mirrors flip left-right but not up-down?

Stand in front of a mirror and raise your right hand. The copy raises what looks like its left. Yet your head stays up and your feet stay down. It feels like the glass has chosen an axis — sideways, but never vertical. It hasn't. A flat mirror reverses exactly one direction, and it isn't left-right: it's front-to-back, the depth axis running straight into the glass. As physicist Christopher Baird puts it, mirrors do not flip left to right; they flip front to back. The left-right swap is something your own brain adds afterward.

What the glass actually does

A mirror obeys one rule: the angle of reflection equals the angle of incidence. Light leaving your chin bounces straight back at chin height. Light from your left shoulder returns on the left. Nothing crosses over. The only change is depth — the point of you nearest the glass becomes the point of the image nearest the glass, so the copy appears to face back out at you. Up is untouched. Sideways is untouched. One axis reversed, two preserved. That's the entire optical operation.

Where the swap really happens

A depth-reversed copy of a person is a strange object — it doesn't match anything in your visual library. So your brain quietly models it as the nearest familiar thing: a real person who walked around behind the glass and turned to face you. But to compare yourself with that imagined turned person, you rotate them in your head — and you rotate around the vertical axis, the way people actually turn. That rotation is what exchanges left and right. The mirror reversed depth; your mental turn converted the depth reversal into a left-right one and pinned the blame on the glass. Turn around a horizontal axis instead — lie under a mirrored ceiling with your feet toward a wall — and the same mirror now seems to flip you head-to-toe while leaving left and right alone. The flip follows your chosen rotation, not the mirror.

The inside-out test

Here's the strange part: reversing depth alone is enough to change handedness. Push a right-hand rubber glove through itself — inside out, no spinning — and it becomes a left-hand glove. That is precisely the operation a mirror performs on your whole body in light. The reflection isn't a rotated you; it's an inside-out you. No rotation in ordinary space can turn one into the other, which is why your mirror image can never be made to overlap you exactly — and why the swap feels so stubbornly built into the glass when it was your idea all along.