Why is an image laterally inverted in a plane mirror?

Answer 1

I believe that this is a strong and compelling psychological illusion. It appears so because you, a laterally symmetrical object, are looking into the mirror. The mirror is reflecting your symmetry, and in the way that plane mirrors do it. What's more, gravity orients you in a certain way, as it does most laterally symmetrical things. The illusion is about symmetry and gravity.

Make Some ObservationsTake a full photograph of anyone, standing up and head to foot, and hold it in front of you so you can see its reflection while you look into a mirror. Turn the photograph 90 degrees with the photo image's head at your left hand. It will appear relative to the photo and its reflection that its 'lateral inversion' is in what you call up-down, but when you look up at your face you will swear that there is no 'inversion' along that dimension. The mirror can't be selectively and simultaneously 'inverting' one image and not the other, and switching its 'inversion' depending on your gaze. Regarding mirror images, maybe 'plane mirror image' is a better term than 'inverted'.

Look again at the sideways photo reflection. You know that the head of the person in the real photograph is pointing toward your real left hand. But which hand is nearest the head in the mirror image? The right hand of your mirror image. So right-left is nothing but a property of your clearly symmetrical body, and up-down are clearly ideas of orientation, or symmetry, with regard to gravity. And the mirror is just hanging there, giving you an ordinary plane mirror image of what is before it. So whatever the plane mirror is doing, it is doing the same thing along any linear axis you can think of.

Answer 2

It really isn't. Things that are up appear up in the image, things that are down appear down, things to the right appear to the right, and things to the left appear to the left. What is really inverted is front and back.