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