Concepts explained
Why Does a Mirror Flip Left and Right? It Never Did
Raise your right hand and the figure raises its left. So why does the mirror leave up and down alone? It does not — and it never swapped left and right either.
A mirror and a wall differ in tidiness, not material
Wallpaper does exactly what the apple does. It takes light in, swallows some, makes new light and sends it out. So does a mirror.
The difference is only how tidily the job is done.
The surface is extremely even, so light leaves at the same angle it arrived. The ordering of the rays survives, and with it the shape. That is why you get an image.
The surface is rough, so light scatters every which way. The ordering is lost and the shape with it. Only brightness remains.
A mirror has never reversed left and right
Raise your right hand at a mirror and the figure raises its left. But then why does the mirror leave up and down alone?
It does not swap left for right. It swaps front for back. Your nose points at the mirror; the nose in the mirror points at you. Left, right, up and down are all untouched.
The sense of a left-right swap comes from you turning yourself around in your head to line up with the figure. People pivot sideways, not by standing on their hands. The one who reversed left and right was you.
The question that remainsIf the mirror reversed nothing, where did the reversal we feel actually happen?