Date: Tue Mar 31 14:01:45 2009 Back to Contents ------------------------------------------------------------------------

Author: Zani, Gerald

Subject: Re: LED photoelectric effect apparatus


This may not be the source of your troubles, but there has been for many
years a problem with the experimental methods commonly published to
interpret the data from LEDs to get Plancks constant.

Garver's TPT article does not cite the following important reference:

Answer to Question #53. Measuring Planck's constant by means of an LED
Roger Morehouse
Am. J. Phys. 66, 12 (1998)

To sum up the problem, you cannot drive the LEDs with one value of

The Garver article says to drive the LEDs at "maximum intensity". What
is "Maximum intensity"?

The Morehouse citation points out this problem, and says it is better to
vary the current from zero to max for each LED, plot all the points,
produce the complete diode curve for each LED and only then to determine
the best slope of this curve.

This may give a better experimental value from each LED?

But again, this may not be the source of your particular trouble?
- JZ

-----Original Message-----
From: [] On
Behalf Of John Welch
Sent: Monday, March 30, 2009 4:46 PM
To: tapl
Subject: [tap-l] 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


John Welch

Cabrillo College Physics Dept.

From Tue Mar 31 14:01:45 2009