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Why does a mirror reverse left and right, but not up and down?


Stand in front of a mirror and note that the positions of your left hand, right hand, head, and feet all correspond with their reflections. A mirror does not reverse up, down, left, or right! It reverses the direction of front to back.

The misconception that mirrors reverse left and right arises because when we stand and face other people, our left and right sides are reversed from theirs. Gravity compels us to orient about a vertical axis, and bilateral symmetry makes our left and right sides look the same. As a result, the "common sense" belief is that the everyday reversed appearance of other people is the correct one, and thus the reflection in a mirror must be "wrong".

ISS Astronauts

If we always looked at other people by standing on our heads, that would be normal, and a mirror would appear to reverse top and bottom, but not left and right.
[Photo courtesy spaceflight.nasa.gov]

Reversing Mirror

This special mirror really does reverse left and right!

First published May 2011
Valerie Paulsen