PHYSC 3622
Experiment 2.2
6 February, 2016
Photoelectric effect
Purpose
In this experiment, you will investigate the photoelectric effect and the relationship of
Planck's constant and the work function of a material to the current generated as light is
absorbed by a conductor.
Background
The potential energy of electrons in a conductor is lower than that of electrons outside
the conductor (otherwise, there would be nothing to prevent the electrons from
escaping the material). Because potential energy is relative rather than absolute, we can
arbitrarily set the zero of the electron's potential energy scale to be that level which
exists just outside the conductor. This level is often called the vacuum level. The
potential energy of the electrons in the conductor is consequently negative with respect
to the vacuum level; the energy difference is known as the work function, W , of the
conductor. This name arises because it is the amount of energy (i.e., work) which must
be imparted to an electron in order for that electron to escape the conductor. The work
function of a conductor is a property of the material, and is fairly constant with respect
to temperature and other external influences.
If the conductor is exposed to light, some of that light may be absorbed by the electrons.
If the energy carried by an individual photon is large enough, an electron can gain
enough energy to escape the conductor. Since the energy of a photon is related to its
frequency by E  h , where h is Planck's constant and  is the frequency, we can see
that an electron will have sufficient energy to escape the conductor if h  W . The
light-induced emission of electrons from a conductor is the photoelectric effect.
If we place a second conductor near the original conductor, the electrons ejected from
the original conductor can travel to the second conductor, creating a photocurrent. In
this situation, the original conductor is called a photoemitter or photocathode, and the
second conductor is called a collector or anode. If we apply a potential difference V ,
between the two conductors (using a battery, for example), the electrons emitted from
the photocathode will be accelerated or decelerated (depending on the polarity of the
potential difference) as they travel to the collector. If a sufficiently repulsive potential is
applied to the collector, the electrons can be prevented from reaching it altogether.
Since the kinetic energy imparted to an electron accelerating through a potential
difference V is qV , where q is the charge of the electron, we see that a photocurrent is
observed only when qV  W  h , where the sign of V is taken to be positive when the
potential at the collector is attractive. Thus, we expect to find the relationship between
photocurrent and potential to look something like Figure 1. The potential required to
eliminate the photocurrent is the stopping potential.
In reality, however, the curve looks more like Figure 2, because when the potential
difference is sufficiently repulsive, the photoemitter and collector exchange roles, and a
reverse photocurrent flows. In this case, the stopping potential may be estimated by
finding the intersection of straight-line extrapolations from both parts of the curve.
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PHYSC 3622
Experiment 2.2
6 February, 2016
Figure 1. Ideal photocurrent vs. stopping potential curve.
Figure 2. Observed photocurrent vs. stopping potential
curve.
Procedure
You will use a mercury vapor lamp as the light source in this experiment. Mercury
vapor emits light at a number of discrete wavelengths from the near ultraviolet to the
near infrared portion of the electromagnetic spectrum. To select light of a particular
wavelength, you will use interference filters.
First, turn on the stopping potential power supply and the meters. Next, turn on the
mercury lamp. To turn on the lamp, record the position of the polarity switch on the
front panel (either + or –). Move the switch to the AC position and turn the power
supply on. The lamp will flicker for several seconds; when it stabilizes, move the
polarity switch to the other polarity (that is, if it was in the – position previously, move
it to the + position, and vice versa). Allow the instruments to warm up and stabilize for
at least ten minutes.
Insert a filter into the filter holder between the light source and the phototube (start
with the 365.0 nm filter). Using the potentiometer to vary the potential difference
between the photoemitter and collector, record a current vs. voltage curve. Be sure to
record a sufficient number of points that you can extrapolate the two portions of the
curve to find the stopping potential. Repeat this procedure for each of the remaining
wavelengths (you may find that there is insufficient light intensity at 690.4 nm to
observe any measurable photoelectric effect).
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PHYSC 3622
Experiment 2.2
6 February, 2016
Using Sigma Plot, plot the stopping potential as a function of photon frequency.
Determine the best-fit straight line through your points. The slope of this line is related
to Planck's constant, while the y-intercept is proportional to the work function of the
photoemitter.
Questions
How does your fitted value of Planck's constant compare with the published value?
The photo-cathode used in the experiment is made of an alloy of Sb and Cs on a Ni
substrate. How does your fitted value of the work function compare with those for S b
and Cs? (You can find the work functions of typical metals in Solid State Physics by N.
W. Ashcroft and N. D. Mermin, p.364.)
What are the potential sources of error in this experiment? (Be sure to consider the
possible errors in each stage.)
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