text: why does a mirror reverse left and right and not up and down?   specular reflection returns the image unaltered, what the mirror is really doing is reversing front and back.  imagine you are behind some object reflected in a mirror, the farthest point from you is the objects front, this same front is the closest point to you in the reflection.  now if you think about it we define top and bottom in terms of a fundamental physical law, gravity, right and left on the other hand are not determined this way.  we assign them based on perspective, so when you reverse something in the direction of depth you have to reverse something in the horizontal direction as well.