Why do objects appear reversed left and right in a mirror but not up and down?
The philosophers have only interpreted the world in various ways; the point, however, is to change it. (Karl Marx, 1845)
Umm… I don’t know if you’re serious, but I’ll assume you are. Think about it this way. Stand in front of a mirror. Now move your left hand toward the mirror until it touches the mirror. Did you notice any discontinuities (did the image of your hand jump from right to left)? Conclusion, things are not reversed. Your left hand is still your left hand - your right hand is still your right hand. Another way to look at it is like shadows. When you stand in front of a light and move your left hand, the left hand of your shadow also moves.
What is counterintuitive for you is that you normally think of looking at people face to face and realizing that their hand on your left is their right hand, due to rotation. This is not what is going on with your image in a mirror, however - there’s no rotation.
Hope that answers your question.
To put it another way, when you turn around to face someone behind you, you turn left-right. If, whenever you wanted to talk to the person in back of you, you did it by standing on your head, then you’d be asking, “Why does a mirror reverse up and down and not left and right?”
I know it sounds like I’m being silly, but that really is the answer.
John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams