Why does a mirror seem to swap left and right?

In a mirror, your head stays above your feet. Why, then, do we say that left and right are exchanged? Start with a familiar gesture: raising one hand. If that hand is on the left side of your visual field, its image appears on that side too. The everyday question therefore contains a distinction worth making: are we describing a visible position in the room, or assigning left and right to the person we imagine in the reflection? [1]

For a plane mirror, the geometry presented in Purdue University’s lectures places the virtual image as far behind the mirror as the object is in front. Virtual means that light appears to come from that position: there is no second person behind the glass. The mirror reverses the direction perpendicular to its surface, or depth. Positions parallel to the surface, including the directions towards the top and bottom, are not simply exchanged with one another in the reflection. [2][1]

Takano and Tanaka distinguish this optical effect from the processes through which we recognise and describe an image. Imagining a person turning around to face us introduces a change of orientation that a reflection does not perform as a real body would. Separating the two steps is useful: where is a point in the image, and what do we call that side of a person imagined facing us? The words left and right always require a frame of reference. [1]

Try this editorial exercise without letters to decipher: wear a bracelet on one hand, raise it and follow the bracelet with your eyes. It stays on the same side of the visual field. Then ask which hand you would assign to the figure if you regarded it as a person turned towards you. Any change in your answer concerns the comparison you choose. This does not show that the brain always makes a mistake, or that one habit explains every reversal phenomenon. [1]

The distinction matters when the apparatus changes too. The Exploratorium describes an exhibit using pairs of mirrors at right angles to produce a view different from that in a single plane mirror. We should not extend the bathroom experiment to every reflecting system. In our explanation, a second reflection is not merely decoration: it changes the path of the light. The illustration here is deliberately simple, with one plane mirror and an identifiable mark on a hand. [3]

The discovery does not require correcting everyone who says an image is backwards. It offers a more precise question: backwards with respect to which direction and which viewpoint? Next time you adjust a sleeve in front of the glass, follow the coloured mark before naming the side. The reflection does not need to decide which side is your right. We choose the comparison through which we describe what we see, and that choice deserves a moment of attention. [2]