PURPOSE: To demonstrate the quantum mechanical property that energy is a function of frequency, not intensity, and to qualitatively show the photoelectric effect and Planck's constant.
DESCRIPTION: Light shines from an incandescent lamp, passes through a
selection of colored filters (red,
blue and green), and strikes a photoelectric cell. The frequency of light incident on the emitter
of the photocell causes
photoelectrons to be ejected and pass through a vacuum to a collector. The photocurrent and
photo voltage are be measured by an electrometer.
A variable reverse bias stopping potential is applied to the cathode ring until the
current ceases. This provides a measurement of the stopping potential, V, which is
displayed on a voltmeter. The stopping potential can be measured for the three frequencies
of light above by inserting the three filters. The stopping potential is observed to
increase with the frequency of the incident light.
The use of a phototube, which has a vacuum and contains a collector, is a method
used to overcome these problems. The 1P39
phototube has a vacuum that prevents ionization, and it has a that collector pulls the electrons away as they are ejected by light that strikes the
emitter. Once the electrons in the phototube are liberated they cannot recombine with the positive charge that remains on the zinc surface. See the previous demo 7A10.00
Photoelectric Effect with 1P39 Phototube.
A plot can be made of the stopping potential as a function of frequency, from which the
value of Planck's
constant h can be determined: h=e(V2-V1)/(f2-f1), where e is the charge on the electron, V
is the stopping
potential and f is the frequency of the incident light.
EQUIPMENT: Photocell, two electrometers, light source, power supply and VOM, as photographed.
SETUP NOTES: 1/2 hour.
Green Filter
Red Filter
Blue Filter
