十万个为什么·科学阅读30篇（2）
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Why Do Mirrors Reverse Left and Right, 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 raise your right hand, the mirror image raises what looks like its left hand.
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This happens because mirrors don’t actually flip left and right — they flip front to back.
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Light travels straight from your nose to the mirror and bounces back toward you.
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So the reflection reverses depth, not horizontal or vertical orientation.
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If you lie on your side, the mirror will seem to swap up and down instead.
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Your brain interprets the reversed depth as a left-right swap because you rotate to face others.
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Try pointing directly at the mirror: your finger and its image point at each other, not sideways.
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No mirror can reverse top and bottom unless you change your own position relative to it.
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Understanding this helps clarify how optical symmetry works in everyday life.
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It also shows why written text appears backward — the reversal is about direction, not labels.