Short answer: a flat mirror does not contain a special left-right switch. The reflection reverses the direction perpendicular to the mirror: points in front of the glass appear the same distance behind it. Up stays up and the physical side that is on your left remains on that side of the mirror.
So why does writing look backwards?
Imagine transparent writing held toward a mirror. The mirror image keeps each point aligned across the glass, but the side of the writing that faced the mirror now appears to face you from behind the glass. To compare that image with normal writing, we instinctively imagine rotating the page—or ourselves—around a vertical axis. That mental rotation swaps left and right, which makes the mirror seem responsible.
A quick test with your hands
Raise your right hand while facing a mirror. The image raises the hand on the same side of the room as your right hand. But because the reflected person appears to face you, you label that image-hand as the other person's left. The geometry of reflection did not choose a horizontal axis; our body-centered labels did.
Quick follow-ups
FAQ
Do mirrors literally reverse left and right?
No. A plane mirror reverses the front-back direction perpendicular to its surface; left-right reversal is an interpretation that comes from comparing orientations.
Why does text look reversed in a mirror?
The front-back reversal changes the handedness of the image. When we mentally rotate the writing to compare it with normal text, that becomes a left-right-looking reversal.
Would a horizontal mirror reverse up and down?
The same rule still applies: the direction perpendicular to the mirror is reversed. With a horizontal mirror, that direction is vertical, so the effect can look like an up-down reversal.
Check the evidence
Sources
- Left-to-Right Reversal — Purdue University Physics Demonstrations
- Why do mirrors flip left to right and not up to down? — West Texas A&M University
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