Date: Mon Mar 30 16:58:55 2009 Back to Contents ------------------------------------------------------------------------

Author: John Welch

Subject: LED photoelectric effect apparatus


Hey all -
we're making several homemade PE experiments using LED's, based on the
article by Wayne Garver in the May 2006 Physics Teacher.
For our phototubes, we 'borrowed' two1P39 tubes from our old Pasco PE
experiments, and bought a third one from a surplus distributor. With the
Pasco tubes, we get a nice value for Planck's constant. When we use the
surplus tube with everything else the same, we still get a nice straight
line for our stopping voltage vs frequency graph, but the slope is about
25% too low. When we first had this problem, I sent the tube back and
got a different one from the surplus guy, but the new one he sent had
the same problem. I'm trying to figure out what could cause something
like this. I would expect differences in the coatings to show up as
different values for the work function, but not to effect the slope.
Does anyone have experience with this, or have an idea as to what might
cause this?
Has anyone tried a different kind of phototube or solid state photodiode?


John Welch

Cabrillo College Physics Dept.

From Mon Mar 30 16:58:55 2009