Why Do Mirrors Seem to Reverse Left and Right?

When you raise your right hand in front of a mirror, the person in the mirror seems to raise a left hand. That makes it tempting to say that a mirror swaps left and right. A flat mirror does not actually do that. The direction it reverses is the direction perpendicular to its surface: front and back. The part of you closest to the mirror is represented closest to the mirror in the image, but on the apparent far side of the mirror. Up and down, and side-to-side positions in the room, remain aligned.

Pointing a hand directly at a mirror makes the geometry easier to see. Your real hand points toward the surface; the apparent hand points away from it, behind the surface. That is a front-back reversal. A mark placed on the left side of an object remains on the left side of its image as seen in the room. If a mirror physically exchanged left and right, a sticker on the object's left would have to move across the image to the right. It does not.

Why, then, does left-right reversal feel so obvious? We usually interpret the reflection as another person facing us. To imagine standing where that person stands while facing the same direction as the real person, we mentally rotate a body by half a turn around a vertical axis. That rotation makes the real person's right correspond to the facing person's left. The mirror supplied the front-back reversal; our comparison with a face-to-face person supplies the feeling of swapped handedness.

Reversed-looking text is related to the same geometry. We normally view the printed front face of a page. When that face is turned toward a mirror, its image has the front-back relation reversed, as if the visible writing were being viewed from the opposite side of its plane. The direction in which a word is read consequently looks wrong. This does not require a tiny physical conveyor belt inside the mirror moving every letter from left to right; it follows from reflection across the page-and-mirror geometry.

There is nothing privileged about up and down. Put a mirror flat on a ceiling or floor and view it from above or below, and the direction perpendicular to the mirror is now vertical. The reflection then reverses what we would call up and down. The rule is always the same: a flat mirror reverses only the coordinate direction normal to its surface. We are simply most familiar with a vertical mirror placed in front of us, where that direction is front and back.

Physically, the mirror sends light away at the same angle at which it arrived. Your eyes trace those reflected rays backward in straight lines and locate a virtual image behind the surface at the corresponding distance. The result is a symmetric placement across a plane, not a rotation performed by the glass. That distinction explains why a mirror can preserve the room's left-right layout while still producing an image that is not superimposable on the object.

Mirror left-right reversal is therefore a useful everyday phrase but an incomplete physical description. Compare your left with the image's left and the sides match; compare yourself with a person facing you and they seem to oppose. Next time you use a mirror, move a finger toward and away from it rather than only sideways. The front-back reversal becomes the clearest part of the reflection.