Why does a mirror reflect an image horizontally and not vertically?
It's a popular trick question... A mirror reflects photons, not images.
Thinking of many tiny bouncing photons rather than a flat image helps explain this mental puzzle.
In a well lit room a mirror bounces away photons at all angles all of the time, but only photons that bounce toward your eye reach it, forming the image in the retina and visual cortex.
Photons reflected from your left hand striking the mirror near your right hand bounce away from your eye and so you don't see them.
The photons probably have changed direction a few times along the way before hitting your eye, but there never was an image until it was formed in your retina and visual cortex.
Karl Lilje