Yesterday I taught the basics of geometric optics to my Physics 202 students. We did plane mirrors, spherical mirrors, and thin lenses. All told it’s a fairly straightforward chapter that’s all theme and variations on one equation. The sign rules for object and image distances have to be remembered correctly, but that’s not so difficult.

Now while students aren’t necessarily going to come into class with previously built intuition for Ampere’s law and all the rest of the fun equations of the electromagnetic fields, they do have many hundreds of hours of experience looking in mirrors. All of them know how mirrors work. Or at least I’m sure they all think so!

But even simple things can have subtle complications. I’m thinking about giving this as a quiz question:

*Why do mirrors reflect right and left, but not up and down?*

Give it a try!