十万个为什么·科学阅读30篇（4）

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镜子为什么左右颠倒，却不上下颠倒？

When you face a mirror, your reflection appears flipped horizontally, not vertically.
This happens because mirrors reflect light straight back—not sideways or upside down.
If you raise your right hand, the image raises the hand on its left side.
But that’s only because you mentally rotate yourself to face the same direction as the image.
A mirror actually reverses front and back: it sends light back along the same path it came from.
Try labeling your shirt with 'L' and 'R'—the mirror shows both letters backward, not swapped.
You’d get true left-right swap only if you turned around like a page in a book.
That mental rotation creates the illusion of sideways reversal, even though no horizontal flip occurs.
So the real answer lies in how we interpret depth, not in the mirror’s optical behavior.
Understanding this helps clarify many common misconceptions about reflection and symmetry.