Why Do Mirrors Reverse Left and Right but Not Up and Down?

I love brain teasers, especially when they’re based on real-world things we see every day but maybe don’t really think about.

I first heard of this one not too long ago: Why do mirrors reverse left and right, but not up and down? When you face yourself in the mirror, left and right are reversed, but you’re still right side up. Why?

The answer may not seem obvious because the question itself frames the problem incorrectly! It makes you try to solve the problem starting from the wrong premise. As it happens, Physics Girl has it covered:

Her use of written words and clothes is pretty clever, but the arrow was really good. That’s what makes it all clear: Mirrors don’t reverse left and right, they reverse in and out. Once you see that it all makes sense.

I really like demos like this; this connects physics to everyday life and also shows that how you frame a problem is in many ways just as important as how you think about a problem.

I met Dianna, aka Physics Girl, last year at Comic-Con; by coincidence I had just seen one of her videos and liked it. You should watch them; she has a knack for explaining things simply and well. I especially recommend the video about vortices in pools; it’s really cool, and these stable swirls of water are really fun to watch. I’ve spent some time in pools (and yes, in the bathtub) creating them and watching them propagate. They seem simple but are surprisingly complex phenomena, especially in how they interact with light.

And why not, here’s the video:

I’d suggest subscribing to her videos, and checking out her website, too. In my opinion, we can’t have enough good people out there communicating fun science. If you know of more, leave a comment below! I’m always looking for that sort of thing.