  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 the Planck's constant and e is the
electron charge. The experience is based on the method developed by Millikan
for his measurements on 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, the stopping potentials are plotted
as a function of the frequency of the incident light, and a linear fit is
performed. The slope and the intercept of the line allow respectively to
evaluate the ratio h/e and the ratio W/e, where W is the work function of the
photocathode.
