Today was an oral assessment day in my Physical Optics course. I’ve written about these before, but here’s the short version: Students are assigned a random standard (chosen from the ones that are both active and older than two weeks so that they’ve turned in one assessment already) one class period ahead of time. They are allowed to prepare a single sheet of paper that we then project using a document camera. For each student, I stare at the page for a minute or so and then I begin to ask questions. That continues for about 10 minutes, and then the class discusses the score the student should receive. Ok, not so short, sorry.
One concern I had about how this course was going was that the students weren’t seeing enough connections among all the material (3 chapters so far). They were doing reasonable well on the standards, but it seemed that they weren’t using common language among the various ideas. Today, in class, I was able to deal with that in a relatively low stress environment, especially compared to the other oral exams going on this week in my department (we use oral exams in every course beyond general physics). When students were up talking about calculating complex refractive indices, I was able to lead them in a discussion of which assumptions must be true for the examples they were working. It forced them to think about all the standards at once, and it was great because the rest of the people in the room were able to speak to different standards.
Here’s an example: One student was going through the Lorentz model of refractive index:
and she was talking about how we know that the system will ultimately oscillate at the driving frequency. She was carefully referencing this applet showing how, even when you’re exciting a mass/spring system off resonance, it’ll still ultimately obey the driving frequency. I fixated on her word “ultimately” and asked what happened with the very first portions of the light field. She didn’t really have an answer, but I really like how these oral assessment sessions go because it wasn’t all that weird to lead her into a conversation about Sommerfeld precursors.
I asked her to look carefully over her own page and to consider all the information that chapter two had to say about the very first portion of the light field. She and I, and, really, at this point, the whole class, began to realize that the notion of linearity was crucial to all of our results. All I mean by linearity here is that the polarization of the material (how the electrons move) is proportional to the applied electric field (from the light). Linearity also assumes that all elements of the medium are oscillating at the same frequency of the light field. My student had really set us up to then question what happens at the very beginning because, as she noted, the system only “obeys” (my word) the driving field after a small amount of time, that I lovingly refer to as the “herky-jerky” period. So then we carefully looked at her equations and asked if all bets were off for that portion of the light. Specifically, I asked her if her predictions of absorption (for non-zero ) would go out the window for the initial light. She didn’t want to believe it, but it was fun for me to let them know that it had a name: the Sommerfeld precursor, and that it really causes headaches for the Navy, expecially when they’re trying to build shielding around various nuclear reactors. Basically, shielding like lead works great, but only after the lead starts to oscillate at the radiation frequency. There’s always a little bit of radiation that gets through before the medium settles down and obeys.
Later in the day a few of the students and I were talking about how that was a fun discussion during the class. Imagine that, considering an oral assessment day to be fun! Plus, who knew that something out of chapter 7 of Jackson would ever make students smile!