Today, there are two things in store.
1) First, a short recap of what's cool about lasers, since I went a bit long last time ... associated with that, we're going to measure a laser's wavelength and angular beam divergence in the classroom. Seriously. Don't sit in the back if you want to hide, we'll need the whole room ;-)
2) After that, we'll return to electrons in a periodic crystal lattice and figure out how the crystal geometry alters things like conductivity. As it turns out, the problem will reduce to a combination of our two usual approximations: springs and free electrons. This will lead us nicely into more detailed semiconductor physics.
Finally, if you have any suggestions for a special topic next week, now is the time to suggest ... I'll need the weekend to learn it myself & make up a lecture, depending on how crazy the winning submission is.