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镜子为什么左右颠倒，却不上下颠倒？

When you raise your right hand, the mirror image raises what looks like its left hand.
This happens because mirrors don’t actually flip left and right — they flip front to back.
Light travels straight from your nose to the mirror and bounces back toward you.
So the reflection reverses depth, not horizontal or vertical orientation.
If you lie on your side, the mirror will seem to swap up and down instead.
Your brain interprets the reversed depth as a left-right swap because you rotate to face others.
Try pointing directly at the mirror: your finger and its image point at each other, not sideways.
No mirror can reverse top and bottom unless you change your own position relative to it.
Understanding this helps clarify how optical symmetry works in everyday life.
It also shows why written text appears backward — the reversal is about direction, not labels.