edit: Thank you to all who answered! I’m amazed at how many ways you came up with to answer this.

  • AbouBenAdhem@lemmy.world
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    2 months ago

    It reflects your image across the plane of the mirror. This isn’t physically possible with real-world objects, so your brain substitutes the closest real-world transformation that would approximate the same appearance—which is rotating bilaterally symmetric objects about their axis of symmetry. With a vertically-oriented human, that means turning around so your left is on the right and vice versa.