Why does a mirror seem to reverse left and right?
The short answer
A mirror does not swap left and right. It reverses the direction that points straight through the mirror, from front to back. Because we picture the reflection as a person facing us, left and right seem to switch.
What direction does a mirror reverse?
Light bounces off a mirror. This bounce is called reflection. When the reflected light reaches your eyes, the object seems to be behind the mirror. With a flat mirror, the image appears as far behind the mirror as the object is in front.
A mirror reverses only the direction that points straight toward or away from its surface. Moving toward the mirror becomes moving away from it. Positions along the mirror, such as up, down, left, and right, stay in place.
Why does it seem to switch left and right?
Raise your right hand and imagine someone standing face-to-face with you. That person’s left hand is on your right. When we picture the reflection as a person facing us, it is easy to feel that the hands have switched sides.
But the mirror has not turned the person around. It has only reversed the direction toward and away from its surface. We feel a left-right switch because we compare the image with a person facing us.
Why does up and down stay the same?
An ordinary mirror stands upright. Up and down, like left and right, run along its surface. Only front and back point straight through it, so the image looks upright. If you lay a mirror flat and look down at it, up and down can seem reversed instead.
Why does writing look backward?
Hold a written page with its front facing the mirror, and the writing looks backward. The image is arranged as if you were looking at the page from its back. This is another effect of reversing front and back, rather than swapping left and right alone.