Name:
Physics 200 Tutorial 8:
Photons and the Photoelectric Effect
ln thís tutorial, you will perform a simulated experiment to test
Einstein's prediction for the photoelectric effect, and also work
through the derivation of another important predictÍon of the
photon picture of light known as the Compton Effect.
The basic setup for the photoelectric effect is shown. A metal is
illumínated with light (or other electromagnetic radiation). For
short enough wavelength, it is observed that electrons are
emitted from the metal. lf the wavelength is too long, no
electrons are emitted regardless of intensity. Einstein proposed
an explanation based on the photon picture of light. The central
points are:
-due to the Coulomb attraction of the positively charged
nuclei, there is some minimum energy W required to remove
an electron from the metal
-electrons are liberated from the metal in events where
single photon transfers energy to an electron
-no electrons will be liberated unless the energy of the
photons is greater than W, so we must have hf = hc/À > W
a
These observations explain why no photons are emitted if the
wavelength is too large, but to really test the model, we want
to make a quantitative prediction and verify it experimentally.
Einstein pointed out that if the incident photons have
energy E=hf, and if there is a minimum energy W required to
liberate an electron from the metal, the most kinetic energy
that an electron could have would be
EK'u' = hf-W
Since h and W are constants (for a given metal), this predicts
a
linear relationship between maximum electron kinetic energy
and frequency.
ln order to actually measure the maximum electron kinetic
energy, Millikan came up with this experiment:
,\eaSq.t
Ct
t"erl
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LI6HT
Here, a battery is used to create a potential difference between
the metal on the left side and the metal on the right side (which
are connected in a circuit). ln order to make it to the right side
(and flow through the wire back to the original metal plate) the
kinetic energy of the emitted electron must be at least eV,the
difference in potential energy for the two sides.
etllargerthan EKrr*, no electrons are able
to pass, so no current will be observed in the wire. Thus, the
As soon as we make
maximum kinetic energy of the electrons is equal to e, the
electric charge, times the voltage at which the current stops
EKr.* = êVsþp
Question
1
ln this question, you will do a simulated photoelectric effect
experiment to test the photon picture of light. you can find the
photoelectric effect simulation here:
http://phet.colorado.edu/simulations/sims.php?sim=photoelectric
Effect
(or Google "PHET photoelectric" or get it from my memory stick)
a) Based on the photon model, what is the predicted
relationship between the stopping voltage %top and the líght
frequency?
b) Make a graph of this relationship below. lndicate on your
diagram the places where the curve intersects the V and f axes
(in terms of.W and h). Calculate the predicted slope.
t/:àr
-
T'=çt*n-raJ.,
e= I.6xl.-t1 C
c) Now, choose the metal sodium as the sample. What is the
maximum wavelength light that can eject electrons?
d) What is the work function W for sodium?
e) Fill in the following table:
Frequency (10"H2)
Wavelength(nm)
Stoppine Voltaee
193
244
350
538
750
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f) Plot the results below, and estimate the slope (in the
where wavelength is small enough to eject electrons).
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