Minute Physics: Why Do Mirrors Flip Left & Right (but not up & down)? (Youtube, 3:23min) “This video is about why words flip left & right (aka horizontally) in a mirror but not up & down (aka vertically). The answer has to do with specular reflection, mirrors being like windows into another world (alternate universes, just with in and out flipped!), and transparency of the things we write on.”
Probably the best explanations I’ve seen/heard about mirrors so far.