Why do mirrors flip left and right but not up and down?

A: This classic question has a surprisingly interesting answer. The fact is mirrors don’t actually flip left and right, much less up and down. Instead, they flip the image of the world you see front to back.

When you stand in front of a mirror and raise your right hand, your reflection raises what appears to be its left hand. But that’s only because your reflection is facing you. If someone else stood facing you and raised their right hand, that would also appear on your left side. So the mirror isn’t flipping left and right. Instead, it’s flipping depth: what’s in front of you becomes what’s behind you in the mirror.

The confusion comes from how our brains interpret the reflection. We imagine ourselves turned around to face the same direction as our reflection. In doing so, we mentally swap left and right. But the mirror hasn’t done that: it has simply reversed the image along the axis perpendicular to its surface.

Thus, mirrors don’t prefer left-right or up-down. They only reflect whatever is in front of them, flipped front to back