Does a Mirror Really Reverse Left and Right?

A mirror does not selectively reverse left and right. Following the law of reflection, a plane mirror forms an image of each point at an equal distance behind the mirror. Your head remains above your feet, and a hand on the eastern side of the room does not move to the west in the image. What changes is the front–back direction perpendicular to the mirror: you stand in front of it, while the image appears behind it.

Why, then, does writing on a shirt look laterally reversed? We usually interpret the image as “another person who has turned around to face me”. If a real person turns from facing the same way as you to facing you, they rotate halfway around a vertical axis. Their left and right then oppose yours, while up and down do not change. The mirror has not performed that turn; the frame of comparison creates the impression of left–right reversal.

Try a simple, safe observation: face a mirror and raise your right hand. The reflected hand remains on the same side of the room as the real one. It becomes “the other person’s left hand” only when you treat the image as someone facing you. It is therefore more accurate to say that a mirror reverses the axis perpendicular to its surface. The example also shows why spatial descriptions depend on a frame of reference: room directions, body directions and the viewer’s directions can produce different verbal accounts of the same image.

https://www.physicsclassroom.com/tutorial/reflection-and-mirrors/plane-mirrors/image-characteristics