We propose a laboratory experience aimed at undergraduate physics students to understand the main
features of the photoelectric effect and to perform a measurement of the ratio h / e , where h is
Planck’s constant and e is the electron charge. The experience is based on the method developed by
Millikan for his measurements of the photoelectric effect in the years from 1912 to 1915. The
experimental setup consists of a photomultiplier tube (PMT) equipped with a voltage divider properly
modified to set variable retarding potentials between the photocathode and the first dynode, and a
set of LEDs emitting at different wavelengths. The photocathode is illuminated with the various LEDs
and, for each wavelength of the incident light, the output anode current is measured as a function
of the retarding potential applied between the cathode and the first dynode. From each measurement,
a value of the stopping potential for the anode current is derived. Finally, th…