Why does the image in the mirror flip left and right instead of upside down? Is this a physical property of the mirror itself or a perceptual illusion of the human brain?

为什么镜子里的影像会左右翻转，而不是上下颠倒，这是镜子本身的物理特性，还是人类大脑的感知错觉？

A public trend signal observed across 1 source.