Why do mirrors flip left and right but not up and down?
They do not flip left and right at all — they flip front and back, and you are the one who mentally rotates yourself to face your reflection.
Simple intuition
The plain reason, in everyday words
Raise your right hand in front of a mirror and the figure opposite raises the hand on your left. It looks like a left-right swap. But watch what the mirror actually does: your hand is on the right and stays exactly on the right, in the same place in the room. Your head stays up, your feet stay down. The only direction that gets reversed is the one pointing at the mirror. You are facing the glass and your reflection faces out of it. The swap you notice appears at the moment you describe the reflection as a person — because to say which is that person's right hand, you imagine yourself turned around to face the way they do, and that turn is what does the flipping. The mirror never moved anything sideways; you did, in your head.
Mirrors reverse left and right.
They reverse the direction perpendicular to the glass, and nothing else. Your right hand appears on the right side of the room in the reflection, exactly where it is.
It happens because our eyes are side by side.
Close one eye and the effect is unchanged, and it appears identically in a photograph of a mirror. It is geometry plus language, not binocular vision.
The mirror is really flipping you top to bottom too, we just do not notice.
It flips exactly one axis. A ceiling mirror flips vertically and does nothing sideways, which shows the reversed axis is set by the mirror's orientation rather than by anything about you.
You could just rotate the reflection to get yourself back.
No rotation can convert a left hand into a right hand. That is the formal content of the effect: reflections change handedness and rotations never do.
It is one of the cleanest cases where a puzzle dissolves once the question is stated precisely — the confusion lives entirely in the word 'flip' and in whose left is meant. It also opens onto chirality, which turns out to matter enormously: two mirror-image versions of a drug molecule can have completely different effects in the body, and one of the four fundamental forces is measurably not mirror-symmetric.
Who worked it out
Plato raises the puzzle in the Timaeus, and it has been used as a teaching example in optics ever since, usually to expose the difference between what an instrument does and how an observer describes it.
What problem forced it
The question became philosophically sharper in the twentieth century, when Immanuel Kant's earlier discussion of 'incongruent counterparts' — objects identical in every measurement yet not superimposable — was taken up as an argument about the nature of space itself.
How it changed since
Handedness moved from puzzle to physics. Louis Pasteur separated mirror-image crystal forms by hand in 1848, founding stereochemistry, and in 1956 Chien-Shiung Wu's experiment showed the weak nuclear force distinguishes left from right, overturning the assumption that the laws of physics are mirror-symmetric.
Chirality in chemistry
Two mirror-image forms of the same molecule can behave completely differently in the body, for exactly the reason a left hand does not fit a right glove.
Why photographs of yourself look wrong
You know your face from the mirror, so an unreversed photograph is the unfamiliar version, and slight facial asymmetries do the rest.
Written for Curio rather than collected from a forum — it is part of the curated corpus that ships with the platform. The references it draws on are listed under Sources.