It's a simple question, but one that may never have occurred to you: why do mirrors seem to flip things horizontally but not vertically? Or, in philosophical-mathematical terms, how can mirrors "choose" to flip an image along the x-axis but the not the y?
That's the topic of a challenging new video from the Youtube series Physics Girl (above).
"If I raise my right hand, the mirrored me raises her left hand, but if I put my hand up in the air, she also puts her hand up, not down. So what's going on?" host Dianna Cowern, a.k.a. "Physics Girl,' says in the video. "The mirror is flipping the image, but it's flipping it in the z-direction, that's what's causing all this confusion. If I could take my body and flip it through itself in the z-direction, I would get my mirror image."
Since our bodies are symmetrical, Cowern explains, we perceive ourselves as being flipped left and right. But we're actually being flipped front to back--like a glove getting turned inside out.
See below for a video of Richard Feynman explaining the mirror paradox, once posed to the famous theoretical physicist by his fraternity brothers.
CORRECTION: An earlier version of this post mislabeled Meg Chetwood, who was a guest star in the video, as Physics Girl's host.