Photoelectric Follies

I spent most of yesterday helping out with an on-campus workshop for high school teachers and students. Seven high school physics teachers and seventeen high school students spent the day doing a half-dozen experiments to measure various physical constants.

I was in charge of having them measure Plack's constant using the photoelectric effect. The actual measurement (made using a PASCO apparatus) takes about fifteen minutes, so I gave each group a quick explanation of the history: Einstein proposed the particle model of light as an explanation for the photoelectric effect in 1905, and nobody liked the idea. People eventually came around thanks to a series of experiments, including a 1916 photoelectric effect experiment by Robert Millikan.

Millikan, however, didn't do the experiment because he liked the Einstein model and wanted to prove it correct. Actually, he hated the idea, but had enough integrity to report the actual data that confirmed it.

I pulled up a copy of Millikan's original paper on the Physical Review Online Archive, and read the first couple paragraphs to illustrate this point:

Einstein's photoelectric equation for the maximum energy of emission of a negative electron under the influence of ultra-violet light... cannot in my judgment be looked upon at present as resting upon any sort of a satisfactory theoretical foundation. Its credentials are thus far purely empirical, but it is an equation which, if correct, is certainly destined to play a scarcely less important role in the future development of the relations between radiant electromagnetic energy and thermal energy than Maxwell's equations have played in the past.

I have in recent years been subjecting this experiment to some searching experimental tests from a variety of viewpoints, and have been led to the conclusion that, whatever its origin, it actually represents very accurately the behavior... of all the substances with which I have worked.

I get a kick out of this as an example of how to say "I think this is a bunch of crap, but damn it, it works," in a scientific paper. The paper itself is a nice piece of work, too. Millikan was nothing if not thorough.

As for our modern version of the experiment, it worked really well. PASCO's apparatus is a little on the black-box side of things, but it gives good and consistent results. I think the largest error among the six groups was about 7%, and two groups got withing 1% of the accepted value for Planck's constant.

Running through the same experiment six times was a little draining, but all in all, it wasn't a bad way to spend a Saturday. The teachers and students all seemed to be having fun, and it was nice to do a little hands-on physics for a change.