AP Physics
Name:
Period:
The ability of some materials to absorb light energy and use it to free electrons that then create a current is called the photoelectric effect. The details of the photoelectric effect were important in motivating Einstein to accept and expand Planck’s idea that light is quantized. In this activity we will look at a simulation of the photoelectric effect and see how the material responds to different light conditions.
We will use the photoelectric simulation applet found at:
http://lectureonline.cl.msu.edu/~mmp/kap28/PhotoEffect/photo.htm
One note, in this simulation the maximum intensity of the lamp is the same for every lamp.
Procedure
Part A – Photocurrent for Different Wavelengths
1. Make sure that the radio button for cesium is chosen. Set the stopping voltage at 0.0 V.
2. Set intensity to 50% and make sure it stays there for the duration of the experiment
3. Set the wavelength to 250 nm. Measure and record the photocurrent produced.
4. Increase the wavelength by 50 nm. Record the photocurrent produced.
5. Repeat step 2 until you are at the smallest wavelength possible
Photocurrent for Different Wavelengths (Part A Data)
6. Graph the intensity of the photocurrent as a function of the wavelength of light falling on the metal:
7. What happens to the photocurrent as the wavelength of the light increases?
8. Why does the photocurrent drop off completely at wavelengths above the cutoff wavelength?

Part B – Photocurrent for Different Intensities of Light
1. Make sure that the radio button for cesium is chosen. Set the stopping voltage in the circuit at 0.0 V.
2. Set the intensity at the lowest possible value (0%)
3. a. Set the light wavelength for 400 nm. b. Increase the intensity of the light by 15% and record the amount of current produced. c. Repeat step b until you reach maximum intensity (100%).
4. a. Reset the intensity at the lowest possible value. b. Set the light wavelength for 500 nm. c. Increase the intensity of the light by 15% and record the amount of current produced. d. Repeat step b until you reach maximum intensity (100%).
5. a. Reset the intensity at the lowest possible value. b. Set the light wavelength for 600 nm. c. Increase the intensity of the light by 15% and record the amount of current produced. d. Repeat step b until you reach maximum intensity (100%).
Photocurrent For Different Intensities of Light at a Variety of Wavelengths (Part B Data)
Intensity
(%)
Current
(A)
Intensity
(%)
Current
(A)
Intensity
(%)
Current
(A)
6. Plot all three sets of data on one complete graph and label each by wavelength.
7. Why does the strength of the photocurrent change with the intensity of the light?
8. How can you explain the difference between the different wavelengths?

Part C – Incoming Photons Frequency vs. Kinetic Energy of Ejected Electrons
1. Set the intensity to 50% and leave it there. Move the wavelength to the cutoff wavelength.
2. a) Set the stopping voltage, V
0
, to zero. b) Decrease the wavelength until a small current appears. c) Increase the stopping voltage, V
0
, until the current becomes zero. Record the wavelength of the light and the stopping voltage.
3. a) Reset the stopping voltage to zero then decrease the wavelength still further b) Increase the stopping voltage, V
0
, until the current becomes zero. Record the wavelength of the light and the stopping voltage. c) Repeat steps a and b for 4 more wavelengths.

0
4. Graph the maximum kinetic energy of the photoelectron as a function of light frequency and find the best-fit equation for the graph.
5. Calculate the work function for the data in each trial. Then calculate the average value of all trials.
6. Compare the equation for your graph to the photoelectric energy conservation equation: ( hf = K max
+

0
)
.
Rearrange the equation so it matches your graph. What variable is the slope? What variable is the yintercept?
7. Read the values for the constants you identified in question 6 from your best-fit equation. Convert them to standard units and compare them to expected values.