Photoelectric Effect
Answer Key
Vocabulary: electron volt, frequency, intensity, photoelectric effect, photon, voltage,
wavelength, work function
Prior Knowledge Questions (Do these BEFORE using the Gizmo.)
[Note: The purpose of these questions is to activate prior knowledge and get students thinking.
Students are not expected to know the answers to the Prior Knowledge Questions.]
1. Suppose you went bowling, but instead of a bowling ball you rolled a ping pong ball down
the alley. What do you think would happen?
Answers will vary. [Most students will guess that the ping pong ball would not knock down
any pins.]
2. Suppose you rolled a lot of ping pong balls at the bowling pins. Do you think that would
change the results of your experiment? Explain.
Answers will vary. [Most students will guess that this would not change the results.]
Gizmo Warm-up
The photoelectric effect occurs when tiny packets of light,
called photons, knock electrons away from a metal
surface. Only photons with enough energy are able to
dislodge electrons.
In the Photoelectric Effect Gizmo™, check that the
Wavelength is 500 nm, the Intensity is 50%, the Voltage
is 0.0 volts, and Potassium is selected. Click Flash the
light to send photons of light (green arrows) toward the
surface of a metal plate encased in a vacuum tube.
1. The blue dots on the metal plate are electrons. What happens when the photons hit the
electrons?
The electrons that are hit by photons leave the surface of the metal and move through the
tube.
2. What happens when the electrons reach the light bulb?
The light bulb glows briefly.
When electrons reach the light bulb they complete a circuit, causing the bulb to glow briefly.
Activity A:
Get the Gizmo ready:
Wavelength and
intensity
 Check that the Voltage is 0.0 volts.
 Check that Potassium is selected.
Introduction: Through the centuries, many scientists have debated whether light is a wave or a
stream of tiny particles. In the 1800s, most scientists agreed that phenomena such as refraction
and diffraction supported the light-as-a-wave theory. However, Albert Einstein’s explanation of
the photoelectric effect showed that light can act like a stream of particles as well.
Question: How does the wavelength and intensity of light affect its ability to free
electrons from the surface of a material?
1. Observe: Click Flash the light with a variety of Wavelength values. What do you notice?
Sample answer: Light with long wavelengths (greater than 530 nm) has no effect on the
electrons. For light with a wavelength of 530 nm or shorter, the speed of emitted electrons
increases as the wavelength of light decreases.
2. Observe: Click Flash the light with a variety of Intensity values. What do you notice?
Sample answer: The greater the intensity of the light, the greater the number of photons that
strike the surface of the metal. This also increases the number of emitted electrons.
3. Form hypothesis: Answer the following questions based on what you have observed so far.
A. Which factor determines how many photons will strike the metal? Intensity
Explain: As the intensity increases, the number of photons increases as well.
B. Which factor determines how much energy each photon has? Wavelength
Explain: As the wavelength decreases, the speed of emitted electrons increases.
4. Investigate: Set the Intensity to 10%. Uses the Gizmo to determine the largest wavelength
that will dislodge an electron from the surface of the metal. What is this value? 530 nm
5. Predict: Set the Wavelength to 540 nm. What do you think will happen if you flash the light
with an intensity of 10%? What will happen if you flash the light with an intensity of 100%?
Predictions will vary.
(Activity A continued on next page)
Activity A (continued from previous page)
6. Test: Click Flash the light with an Intensity of 10% and again with an Intensity of 100%.
What happened? No electrons were emitted in either experiment.
7. Explore: Set the Wavelength to 400 nm. Experiment with different intensities of light.
A. Does the light intensity affect how many electrons are emitted? Yes
Explain:
With the intensity set to 10%, one electron was emitted. With the intensity set to
100%, 10 electrons were emitted.
B. Does the light intensity affect the energy (speed) of the emitted electrons? No
Explain:
The speed of the emitted electrons appeared to be the same no matter what intensity
value was used.
8. Infer: For mechanical waves, such as sound waves or ocean waves, increasing the intensity
of the wave increases both the amplitude (height) of the wave and the energy it carries. In
that situation, a long-wavelength, high-intensity wave should have the same effect as a
short-wavelength, low-intensity wave. How does light behave differently from this model?
Sample answer: Light behaves differently from this model because long-wavelength, highintensity light does not have the same effect as short-wavelength, low-intensity light. Longwavelength light has no effect on electrons, even when shone at high intensity.
9. Think and discuss: How is firing photons at the surface of a metal analogous to rolling
different types of balls at a set of bowling pins? If possible, discuss your answer with your
classmates and teacher.
Sample answer: Photons of light are like balls rolling toward bowling pins. The wavelength
of the photons is analogous to the mass of the bowling ball. Firing long-wavelength photons
at a metal plate is like rolling ping pong balls at bowling pins—no matter how many
photons/ping pong balls are used, there will be no effect. Firing a short-wavelength photon
at a metal plate is like rolling a heavy bowling ball at the pins. Any collision will result in
electrons being ejected (or bowling pins getting knocked over).
Get the Gizmo ready:
Activity B:
Voltage gradients
 Set the Wavelength to 300 nm, the Intensity to
100%, and the Voltage to 0.0 volts.
 Turn on Show voltage gradient.
Introduction: The electrons that are freed from the surface of the metal have a specific amount
of kinetic energy. Faster electrons have greater energies than slower ones. The energy of
emitted electrons is measured by setting up an electrical field that opposes their motion. The
voltage of the field is a measure of its strength.
Goal: Use a voltage gradient to measure the energy of emitted electrons.
1. Observe: Check that Potassium is selected. Click Flash the light and observe the emitted
electrons. Increase the Voltage to 1.5 volts, and click Flash the light again.
How does the electrical field affect the motion of the emitted electrons?
The electrical field causes the electrons to slow down as they move through the tube.
2. Measure: The energy of an emitted electron is measured in electron volts (eV). An electron
with an energy of 1 eV can overcome an electrical field of 1 volt. In the Gizmo, increase the
voltage until you find the highest voltage that still allows the electrons to reach the light bulb.
What is this value? 1.8 eV
It is equal to the energy of the emitted electrons in eV.
3. Gather data: With the Wavelength set to 300 nm, measure the energy of emitted electrons
for potassium, calcium, and uranium. Then measure the same values with wavelengths of
250 nm and 200 nm to complete the table.
Element
Energy of emitted electrons (eV)
300 nm
250 nm
200 nm
Potassium
1.8 eV
2.6 eV
3.8 eV
Calcium
1.2 eV
2.0 eV
3.2 eV
Uranium
0.5 eV
1.3 eV
2.5 eV
4. Analyze: What patterns do you notice in your data?
Sample answer: The energy of emitted electrons increases as the wavelength of the light is
decreased. Electrons emitted from potassium have the most energy at each wavelength.
5. Infer: Based on your data, which element is hardest to extract electrons from? Uranium
Explain: At each wavelength, the electrons emitted from uranium had the least energy.
Get the Gizmo ready:
Activity C:
Work functions
 Set the Voltage to 0.0 volts and select Potassium.
 You will need a calculator and a copy of the
periodic table of the elements for this activity.
Introduction: It is easier to remove electrons from some elements than others. The energy
required to free an electron from the surface of a solid is the work function of the element.
Question: How much energy is required to liberate electrons from a material?
1. Predict: In general, the difficulty of removing electrons increases from left to right across
each row of the periodic table. Look up potassium (K), calcium (Ca), and uranium (U).
Based on their positions in the periodic table, which of these elements do you expect to
have the lowest work function? Which element will have the highest work function?
Lowest work function: Potassium
Highest work function: Uranium
2. Gather data: Use the Gizmo to determine the highest wavelength for each element that still
removes electrons. Fill in the first column below. (Leave the other columns blank for now.)
Element
Wavelength (nm)
Frequency (Hz)
Work function (eV)
Potassium
530 nm
5.66 × 1014 Hz
2.34 eV
14
Calcium
420 nm
7.14 × 10 Hz
2.95 eV
Uranium
340 nm
8.82 × 1014 Hz
3.65 eV
3. Calculate: The frequency of a wave, measured in hertz (Hz), is the number of waves that
passes a point each second. To calculate the frequency (f) of an electromagnetic wave,
divide the speed of light (c) by the wavelength (λ):
f 
c

The speed of light is 299,792,458 m/s, or approximately 3.0 × 1017 nm/s. Using the equation,
calculate the frequency of each wavelength given in the table. Fill in the second column.
4. Calculate: The energy of a photon depends on its frequency. The energy of a photon (E) in
electron volts is equal to its frequency (f) multiplied by Planck’s constant (h):
E (eV) = h·f
In this calculation, h is equal to 4.136 × 10-15 eV·s. Calculate the work function of each
element in the table above. (Note: The values in your table are approximations.)
(Activity C continued on next page)
Activity C (continued from previous page)
5. Draw conclusions: Based on the calculated work function for each element, which element
holds onto its electrons most tightly? Explain.
Uranium has the highest work function, so therefore uranium holds onto its electrons most
tightly.
6. Think and discuss: When the photoelectric effect was discovered, scientists were surprised
that low-frequency light was unable to remove electrons, even when emitted at extremely
high intensities. (In other words, scientists expected the low frequency of the light to be
offset by its high intensity.)
How does thinking about light as a stream of particles, rather than a single wave, explain
this result? If possible, discuss your answer with your classmates and teacher.
Sample answer: If light was a wave, then low-frequency light should be able to liberate
electrons from the surface of a metal if it was emitted at a high enough intensity. If light
consisted of a stream of photons, only photons with a high enough energy would be able to
liberate electrons from the surface of a metal. Raising the intensity of the light increases the
number of photons but not the energy of each individual photon. Therefore, no electrons
would be liberated even if the light intensity was very high.