This week’s lab (well, half of it– the class is so big that I have to run two experiments in parallel) is somewhat controversial, so I thought I would throw this out to my wise and worldly readers to see what you all think.
The problem is this: we have two different set-ups for doing a photoelectric effect experiment. One of these is a PASCO apparatus with the phototube wired to a circuit inside an actual black box. You shine light into the tube, press a button, and the output of the box rises to the stopping potential for that frequency in a more-or-less exponential manner. This gives very nice results, often within 1% of the accepted value of Planck’s Constant.
The other is an old-school lab, using a homemade monochromator and a phototube with an external voltage generator supplying the stopping potential. For each color of light, the students watch the output of the phototube on an oscilloscope, measure the output voltage for a handful of applied voltages, and extrapolate to find the stopping potential. This is much closer to the way the experiments were originally done, but it also tends to give results that differ from the accepted value by 20-30%.
Which version of the experiment would you prefer?
This term, I’m trying to split the difference between the two, by giving the students the PASCO boxes and a collection of physical and optical components, and having them assemble the experiment themselves. This is partly because I couldn’t find the components of the older version in time to do the set-up, and partly because I’ve never really liked the other version.
A few of my colleagues are really enthusiastic about the older version, saying that it really lets students see what’s going on. While there’s some truth to that, the reports on that version tend to resemble “Electron Band Structure in Germanium, My Ass. I’m not convinced that the pedagogical benefits of applying the stopping potential themselves aren’t undermined by the fact that the results almost invariably suck.
It’s a fine line. Had I found the apparatus in time (the monochromators were moved to the deep storage area in the basement last year), I probably would’ve done that version, because it better justifies the three-hour lab period (with set-up done in advance, the PASCO version takes less than an hour). I don’t think I really believe that the added complexity adds anything, though.
I’m curious to know what other people think, though. Would you rather have a black-boxy lab apparatus that gives good results, or a more transparent set-up that barely gets the right order of magnitude? Which would you rather teach? Which would you rather do as a student?