Download Photoelectric Effect Wear Safety Goggles whenever a mercury light

P31220 lab
Photoelectric Effect
Purpose: Students will observe the Photoelectric Effect and measure Planck’s Constant.
Wear Safety Goggles whenever a mercury light is turned on.
The mercury lights emit large quantities of ultraviolet light which is harmful to your eyes. Do
NOT use the mercury lights as reading lights, even if you are wearing goggles.
Introduction: When sufficiently energetic light strikes a solid, electrons are ejected from the
solid. This “Photoelectric Effect” was first observed for metals such as zinc. Today it is also
known to occur with some semiconductor materials. The photoelectric effect is interesting for
several different reasons.

It’s useful. We can use it to control electric circuits with light. It’s used to turn the street
lights on at night and to ring shop bells when customers enter the door.

It’s mysterious. Some details of how the electrons are ejected are still unknown.

It’s historically important. The Photoelectric Effect led to the development of Quantum
Mechanics, which is a major branch of physics. Quantum Mechanics is used extensively in
physics, chemistry, computer science, laser technology, communications technology, and
electronics.
The photoelectric effect was discovered in 1887 by Heinrich Hertz, whose claim to fame was his
pioneering work in understanding radio waves. You may have seen his name in your car. AM
radio frequencies are between 500 and 1400 kHz (kilohertz), and FM radio frequencies are
centered around 100 MHz (megahertz).
The discovery of the photoelectric effect reopened a very old controversy about whether light
consists of particles or waves. Isaac Newton’s opinion, in 1665, was that light consists of
particles. In 1885, James Clerk Maxwell mathematically proved that light was an
electromagnetic wave. We can easily see that light is a wave, because light makes interference
and diffraction patterns. Only waves do that. The energy carried by a wave is related to the
intensity of the wave. Since the light waves were observed to eject electrons from metal,
scientists predicted that the energy of the ejected electrons should depend on the intensity of the
light. But it didn’t. Even though light has all of the behaviors that qualify it as a wave, here is a
phenomenon in which light is acting like a stream of particles. The controversy was finally
resolved in 1905 by Albert Einstein. Is light a stream of particles, or is it a wave? The answer is
that light is a bit of both. This confusing state of things is at the heart of quantum mechanics, in
which matter also exhibits both particle and wave characteristics and where absolute certainty is
replaced by statistical probabilities. Quantum phenomena occur mostly in things that are at the
size of a molecule or smaller. Quantum mechanics may be weird, but you literally can’t live
without it.
In this lab, you will observe the photoelectric effect. You’ll use it to observe the work function of
the metal and measure Planck’s Constant, which is one of the fundamental physical constants.
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P31220 lab
Experiment 1: Computer Simulation of the Photoelectric Effect
Go to http://phet.colorado.edu/en/simulation/photoelectric. Download and run the applet.
When the Java applet opens, you will see a lamp placed next to a photocell. You can adjust the
intensity and color of the light by using slide controls in the applet.
The light shines on an electrode called the “photocathode.” The electrons are ejected from the
photocathode by the light. The other electrode is called the “anode,” regardless of its charge. We
can measure the kinetic energy of the electrons by finding the minimum voltage, called the
“stopping potential”, that will keep them from reaching the anode. The maximum kinetic energy
of the electrons is simply
Eq. 1
where e is the charge of the electron and Vs the stopping potential. The energy of the photons
(particles of light) striking the photocathode is given by
Eq. 2
where f is the frequency of the light in 1/s or Hz (same thing). Note that f = c/λ, where c is the
speed of light in a vacuum (3.00 x 108 m/s) and λ is the wavelength of the light in meters. The
constant h is Planck’s constant.
Eq. 3
You might think that we’re going to make the energy in Eq. 1 equal to the energy in Eq. 2. Not
quite. The energy of the ejected electron will be a bit less than the energy that the electron
absorbed from the photon. It takes a bit of energy to break the electron free of the photocathode.
This is called the “work function” (symbol φ) and it is similar to a binding energy. It is a
physical property of the material from which the photocathode is made. The maximum kinetic
energy of the ejected electrons is therefore given by
Eq. 4a
or
Eq. 4b
Equation 4, shown in two forms, is the main equation that describes the photoelectric effect.
What to do:
Go to the computer applet, then go to the Data Sheet. Design and perform your own
experiments. Answer the Analysis Questions, based on your experiments.
Budget your time. You can access the applet outside of class, but not the equipment. Allow at
least 1 hour for using the Photoelectric Effect apparatus.
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P31220 lab
Experiment 2: Photoelectric Effect in the Laboratory
In this part of the lab, you will use the photoelectric effect to measure Planck’s Constant h.
Please refer to Eq. 4a. The slope of a graph of Vs vs. f is h/e. The y-intercept of the graph is φ.
Apparatus and Procedure for measuring Planck’s Constant:
Thumb
screw
Fig. 1: PASCO Photoelectric Effect Apparatus
The apparatus consists of a mercury lamp attached to the sensor apparatus with a hinged bar.

Put on your safety goggles to protect your eyes from ultraviolet light.

Turn on the mercury lamp. The lamp requires at least five minutes to warm up fully.
CAUTION: The mercury lamp will become quite hot, especially on top.

The grating on the lamp is made so that the spectrum of light is brighter on one side than the
other. Use the brighter spectrum. Arrange the apparatus as follows:
o When you’re sitting in front of the apparatus, put the mercury lamp on your right.
o The white label on the lens/grating (mounted on front of the lamp) should face you.
o The lens/grating’s black thumbscrew should be on top.
o The diffraction grating is made so that the light is brighter on one side than the other.
Use the brightest spectrum. The h/e Apparatus (sensor head) might need to be to the
right of the grating, not as shown in Fig. 1. If in doubt, ask your TA for help.

Attach a DMM to the OUTPUT terminals of the apparatus to measure the stopping voltage.
Make sure that the DMM is set up correctly for measuring voltage. If you forgot how, please
ask for help.

Once the lamp is warmed up, use a piece of paper as a screen so that you can view the
mercury spectrum. You should see a bright yellow line, a green line, and perhaps some blue
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P31220 lab
or violet lines. The front of the h/e apparatus is coated with a fluorescent paint. If you put this
into the light, you should be able to see all five lines. We’ll call them yellow, green, blue,
violet, and ultraviolet. Because of the paint, the blue, violet, and ultraviolet may all appear to
be a blue color on the front of the h/e apparatus. The blue and UV lines might be dim.

Please refer to Fig. 2 and Fig. 3. The front of the h/e apparatus consists of a slit. There’s a
cylindrical shield behind the slit in the white reflective mask. Flip this shield out of the way
so you can view the photocathodes. They look like dark squares against a white background
inside of the apparatus. Adjust the position of the yellow line so that the light passes through
the slit and falls squarely on the photocathodes. DO NOT TWIST THE SENSOR HEAD
without first loosening the thumbscrew! Often it’s better to move the lamp instead of the
sensor head. The top of the lamp is hot, so hold it on the bottom. If you have difficulty with
alignment, please ask for help.
Thumb
screw
Fig. 2: Detail of h/e Sensor Head
Fig. 3: Photocathodes

Flip the cylindrical shield back into place to keep out stray light.

Once everything is positioned, turn on the h/e sensor head. Press and release the “Push to
Zero” button on the side of the apparatus. You may have to hold the sensor head so that it
does not move while you push the button. When the button is released, the DMM reading
should rise to a maximum and remain steady at the stopping voltage Vs. Record Vs. Repeat
this reading a few times to make sure that it is reproducible.

Cover the slit in the h/e apparatus with the appropriate yellow or green filter whenever you
use the yellow or green spectrum lines. The filters ensure that only low energy yellow or
green light strikes the sensor. The blue, violet, and ultraviolet lines don’t require filters.

Take measurements of Vs for each of the five spectrum lines and record them on the Data
Sheet. Graph Vs vs. f and answer the Analysis Questions.
Procedure for Observing Stopping Potential as a Function of Light Intensity:

The intensity filter has several gray stripes. It transmits 100%, 80%, 60%, 40%, or 20% of
the light falling on it. Put some light into the sensor head as before, then cover the slit with
the desired part of the intensity filter. You can choose any spectrum line, but if you choose
yellow or green please use the yellow or green filter with the intensity filter.

Measure and graph Vs vs. intensity. Adjust the scale of your graph so that the y axis goes
from zero to a convenient voltage above Vs. Record your data and answer the Analysis
Questions on the Data Sheet.
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P31220 lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________
Data Sheet and Analysis Questions:
Experiment 1:
1. Select the sodium photocathode. Put the lamp intensity in the middle of its range. Set the
battery voltage to zero. Turn on the graphs, if you like.
a. Slowly change the color of the light from IR to UV. At what wavelength do electrons
begin to be ejected? _______________ What color light is this? _____________
b. What is the photon energy at this wavelength, in eV? ______________ Show your
work below.
2. Select a wavelength of 300 nm.
a. What is the photon energy of this wavelength of light, in eV? __________________
b. What happens when you increase the light intensity?
c. Use the battery. What is the stopping potential of the electrons, in volts? _________
d. Use Eq. 4a or Eq. 4b to find the work function of sodium. Show your work below:
e. What happens to the stopping potential when you increase the photon energy?
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P31220 lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________
3.
Find the work function φ of the other photocathode materials in the applet. Please record
your data and the results of your calculations below:
Photon energy
Photocathode
Material
λ (nm)
φ (eV)
Vs (Volts)
Sodium
Zinc
Copper
Platinum
Calcium
??????
Experiment 2: Photoelectric Effect in the Laboratory
Data for Measurement of Planck’s Constant
Spectrum
Line
Wavelength
λ (nm)
Yellow
578.2
Green
546.1
Blue
435.8
Violet
404.7
Ultraviolet
365.4
Frequency
f
(x1014 Hz)
Energy
(eV)
Vs
Vs
Vs
Reading
1
Reading
2
Reading
3
Vs
Average
δVs
4. You should not use the 2% of the DMM reading as your error unless you can show that this
was the source of the largest error. How did you determine δVs?
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P31220 lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________
Data for Vs vs. Intensity
Spectrum color used: _________________
Intensity
Vs
Vs
Vs
Reading 1
Reading 2
Reading 3
Vs
Average
δVs
20%
40%
60%
80%
100%
Experiment 2: Analysis Questions:
6. Graph Vs vs. frequency, with error bars. Make sure the y intercept is visible on the graph.
a. What is Planck’s Constant, based on your data? _______________________
Figure this out using the slope of your graph. Show your work!
b. What is the estimated measurement uncertainty in your measured value of h?
c. How did you determine your error in h?
d. Does your result agree with the accepted value for Planck’s Constant? Explain.
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P31220 lab
Name: ________________________________
Lab Section: ________________ T.A.’s ____________________________________________
Today’s Lab Partners: ___________________________________________________________
e. Determine the y-intercept of your graph. What is the work function of the
photocathode? _______________
f. According to PASCO Tech Note 303, the photocathode of the AP-9368 h/e
Photoelectric Head is made of antimony treated with cesium vapor. The anode is a
nickel alloy, and the work function is 1.36 ± 0.08 eV. However, the instruction
manual for this apparatus indicates that the work function might be 1.412 eV or
1.459 eV.
Do any of these values agree with your data? Explain.
http://www.pasco.com/support/technical-support/technote/techIDlookup.cfm?TechNoteID=303
http://leona.physics.tamu.edu/Phys327.11s/pasco-photoelectric.pdf
7. Create a graph of Vs vs. intensity, with error bars.
a. Interpret your graph. What does it tell you about the relationship between electron
energy and light intensity?
b. Is your graph consistent with what you would expect after reading the introduction to
this lab? Please explain.
Graphs to turn in: Vs vs. frequency and Vs vs. intensity, both with error bars.
Clean-up: Please turn off the mercury lamp, the sensor head, the DMM, and your desk lamp.
Leave the filters on the base of the sensor head. Dispose of trash and recyclables properly.
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