Back when I was an undergrad, we did a lab in the junior-level quantum class that involved making a dye laser. We had a small pulsed nitrogen laser in the lab, and were given a glass cell of dye and some optics and asked to make it lase in the visible range of the spectrum.
My partner and I worked on this for almost the entire lab period, and got nothing more than the occasional faint flicker of a green beam. We got the TA to help us, and he couldn’t do any better. The TA went to get the professor teaching the class, but he was helping other students with one of the other experiments (this class ran several experiments in parallel), so it took a while, and we kept beating on it to no avail.
Finally, the professor came in, and said “OK, what have we got?” We explained the situation, he did a couple of quick checks of the alignment, and agreed we had set it up right. Then he said “You know, sometimes the orientation of the cylindrical lens makes a difference,” and rotated one of the optics in its holder by about 10 degrees. At which point, in my memory at least, this brilliant green beam shot across the lab like in Real Genius. We had everything lined up about as well as it could possibly be lined up, but were thwarted by a factor we hadn’t even been aware was something we could adjust.
I try to avoid this sort of evil professorial magic trick as much as possible, because I remember the frustration of that afternoon trying to get the dye laser going. Sometimes, though, it can’t be avoided, and damn if I didn’t do it again yesterday.
We have an upper-level lab course in our curriculum, which is taken by junior and senior physics majors. We have this broken up into six different modules where students do some advanced experiments and write them up as formal reports. My turn for this has come around again, so I spent yesterday (which was a beautiful day, I should note) in a windowless basement lab running one of these experiments on the calibration of the free spectral range of a Fabry-Perot interferometer.
A couple of years ago, I got complaints that the lab was too cookbook-y, so I modified it to have the students do more of the alignment themselves, taking one of the mirrors off the Fabry-Perot, and very slightly misaligning the other. That worked pretty well last year, and I did the same thing again this year.
Unfortunately, the laser we’re using this year gives a less distinct pattern than the one we had last year (which dies last week, damn it), and I failed to make clear that the adjustments that needed making were small. So yesterday’s lab group spent an hour looking for a big and brilliant pattern to no avail, in the process working the interferometer off into a regime where it couldn’t possibly work well (more or less). After more than an hour of futility, they came and asked me to help.
Whereupon I did the Evil Professor Magic Trick, and got the thing working in less than half an hour. Which I can do, because I’m an optics guy going way back, and know exactly what I’m looking for, and a whole bunch of tricks to get there more quickly. But as I know from experience, it’s really frustrating to beat on an experiment for a long time, then have a faculty member come in and make it look easy.
This carries on longer than you might expect, by the way. One of the post-docs at NIST when I was there was struggling with a model 599 dye laser from Coherent one day, and went to ask the local expert on these lasers if there was a trick he was missing. The local expert came in, took hold of the input and output coupler mounts in his hands, wiggled them around for a minute and got it lasing, then eyeballed the position of the mirrors, let go of them, and used the screws to get them back into roughly the right position, from which it took only a minute to get the laser up and running.
Both of these guys had Ph.D.’s in physics from doing experiments with lasers, but the local expert had done his thesis work with the model 599 laser with serial number 001. He had forgotten more about how those lasers work than any of the rest of us ever knew.
So, I don’t know if there’s any way to avoid ever doing the Evil Professor Magic Trick– it’s really just a function of vastly different levels of expertise, and teaching is necessarily a situation involving vastly different levels of expertise. There will be times when something goes wrong that only an expert can fix, but that expertise will make it look easy. All you can really do is try to minimize the number of times that happens.
Of course, the real question here is what I should do with next Tuesday’s lab group. Should I give them better instructions to avoid the need for the Evil Professor Magic Trick, or should I try to ensure that their lab experience closely parallels that of yesterday’s group?