Why do mirrors seem to flip left and right but not up and down?

Mirrors do not actually flip left and right or up and down; instead, they reflect the third dimension, which is front to back^[3][5]. When you look into a mirror, the light rays from your body are reversed along the axis perpendicular to the mirror surface^[2][6].

To visualize this, imagine turning yourself 180 degrees to face your reflection^[2]. Because you have rotated your body to match the reflection, your brain interprets the front-to-back reversal as a left-right swap^[2]. This perception is simply a result of how we orient ourselves relative to the mirror^[6].