This experiment investigates the photoelectric effect which is a phenomenon of electrons being ejected from metals when light is incident on them. Scientists in the early part of the twentieth century were attempting to understand this effect by modeling light as a wave or light as a particle to see which model better explained this effect. In this experiment we ask you to answer this question using the ideas of light as waves or particles from the early twentieth century. You can answer this question by answering a related question. “Does the energy of the ejected electrons depend on light frequency or light intensity?” You will study the relationship between the frequency of light incident on a piece of metal to the energy of electrons ejected from that metal. Next you will look at the relationship between the intensity of the light incident on the metal and the energy of the ejected electrons. Ultimately you will decide whether light behaves as a wave or particle in the photoelectric effect.ST. LAWRENCE UNIVERSITY
Physics
The Photoelectric Effect
Name_______________
Introduction
This experiment investigates the photoelectric effect which is a phenomenon of electrons being ejected
from metals when light is incident on them. Scientists in the early part of the twentieth century were attempting to
understand this effect by modeling light as a wave or light as a particle to see which model better explained this
effect. In this experiment we ask you to answer this question using the ideas of light as waves or particles from the
early twentieth century. You can answer this question by answering a related question. “Does the energy of the
ejected electrons depend on light frequency or light intensity?” You will study the relationship between the
frequency of light incident on a piece of metal to the energy of electrons ejected from that metal. Next you will look
at the relationship between the intensity of the light incident on the metal and the energy of the ejected electrons.
Ultimately you will decide whether light behaves as a wave or particle in the photoelectric effect.
Theory
Scientists have a long and interesting history questioning the nature of light. Some experiments suggest light
behaves as a wave and others suggest that light behaves as a particle. How can we understand the photoelectric
effect where electrons are ejected from a metal by light? Which model of light is better suited to interpreting this
phenomenon?
Scientists in the early part of the twentieth century predicted that if light were a wave, the energy of the
electrons ejected from the metal should increase with light intensity; more light intensity should provide more light
energy and more electron energy. They also expected that the light frequency would not affect the energy of the
light and, therefore, would not affect the energy of the ejected electrons.
Max Planck suggested that if light behaved like a particle, it would have an energy proportional to its frequency.
Einstein applied this idea to the photoelectric effect. Higher light frequency should produce higher light energy and
higher energy ejected electrons. Each photon has energy E = hf, where f is the photon frequency and h is Planck’s
constant. When a photon strikes a metal surface, a minimum amount of energy, (the work function), is needed to
knock an electron off. The remainder is given up to the kinetic energy of the ejected electron. This should result in
the following relationship:
Ejected electron energy = light energy – energy required to remove the electron.
Kmax = hf –
By investigating the relationship between ejected
electron energy and light frequency, and the
relationship between ejected electron energy and light
intensity, we can answer the questions laid out in the
introduction.
b
a
Apparatus
Our virtual apparatus has two metal plates that are
connected by a variable battery so that we can change
the potential difference between the plates. There is a
light source that allows us to change the wavelength
c
(color) of light (Figure 1) incident on the left metal
plate. When the photons from our light source strike
the metal plate electrons can be ejected. These ejected
electrons have various amounts of energy, and
if they have enough energy to overcome the
Figure 1 shows our virtual apparatus from PhET. Using the panel
potential difference between the plates, they
on the top right we can control the wavelength (a) and intensity (b)
travel across the apparatus to the other plate.
of the light. The battery on the bottom is variable (c) allowing us
When electrons are ejected, we can adjust the
to change the polarity and magnitude of the voltage between the
battery voltage to make it more negative to stop
plates. There is a current meter to the right of the battery that
them from going to the right-hand plate.
measures when electrons make it from one plate to the other.
©SLU Physics
Revised: 4/4/2020
The Photoelectric Effect
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Physics
Predictions
We expect light to behave as a wave or as a particle.
1. If you have a beam of particles hitting a target, how do you make that beam more intense?
2. According to the wave model, will the light energy increase linearly with intensity OR with
frequency?
3. According to the particle model, will the light energy increase linearly with intensity OR with
frequency?
Use the following two graphs to predict if the energy of the ejected electrons depends on light
intensity according to each model. You can draw on each graph using the line tool: Select the Insert tab
at the top of the document window and click on the “Shapes” icon. Select the line tool from the menu of
shapes. Use this tool to draw a line illustrating the expected behavior in each case below and explain your
reasoning. (You’ll need to select this tool for each line drawn.)
Wave Model
Particle Model
Electron energy
Electron energy
Light Intensity
Light Intensity
Explanation:
Use the following two graphs to predict if the energy of the ejected electrons depends on light
frequency according to each model. Draw a line illustrating the expected behavior in each case below
and explain your reasoning.
Wave Model
Particle Model
Electron energy
Electron energy
Light frequency
Light frequency
Explanation:
©SLU Physics
Revised: 4/4/2020
The Photoelectric Effect
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Department of Physics
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Physics
Experiments:
Experiment 1: The Effect of Frequency
1.
Use Data Table 1 below to record your data and calculations:
Line Color
(nm)
f (Hz)
Vstop (V)
eVstop (eV)
-4.2
-3.2
-2.4
-1.6
-1.2
-0.6
0
2.
3.
4.
Go to the PhET site at this link: https://phet.colorado.edu/en/simulation/photoelectric
Download the Java simulation to your computer; double-click the file photoelectric_en.jar to run.
The simulation allows you to choose from a variety of materials for the left plate. The simulation defaults
to Sodium, so leave it selected in the pull-down menu on the upper right.
5. The “Intensity” is set to 0% when the simulation starts. Drag slider (b) on Figure 1 all the way to the right
until it reads 100%.
6. The slider on the battery can be dragged left or right for negative or positive voltages. Drag slider (c) on
Figure 1 to the left until the stopping voltage reads -4.2 volts.
7. Drag the wavelength slider (a) all the way to the left until it reads 100 nm, to start with the shortest
wavelength of light available.
8. Now increase the wavelength slowly while looking at the current meter. You are looking for the
wavelength where the current just turns to zero. The wavelength adjusts in increments of 2 to 3 nm, so be
sure to set this value as carefully as you can. You’ll notice that increasing the wavelength beyond this point
also results in zero current. You are looking for the ONE wavelength that gives zero current just above the
wavelength that produces current. The simulation does not respond instantly, so give the simulation time to
settle when looking for the zero point.
9. Record the wavelength you found in Data Table 1 above, as well as the color of this wavelength (use the
abbreviation UV for ultraviolet).
10. Repeat steps 6 through 9 for the remaining stopping voltages listed in Table 1.
11. Calculating the Line Frequency: Calculate the frequency of each wavelength in units of Hertz (Hz) using
the equation = = and record in the Table 1. Recall that c = 3108 m/s, and 1 nm = 110-9 m!
Your frequencies should be in the 1014 to 1015 Hz range. Here is an example calculation for 600 nm light:
3 108 /
= =
= 5 1014
600 10−9
12. Calculating the Electron Kinetic Energy: This is very straightforward. The kinetic energy, given by eVstop,
is the same numerical value as the stopping voltage (without the sign), but the units change from V to eV.
Experiment 2: The Effect of Intensity
1.
Use Data Table 2 below to record your data:
Line Color
©SLU Physics
Revised: 4/4/2020
(nm)
Intensity %
350
100
80
60
40
20
The Photoelectric Effect
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Vstop (V)
eVstop (eV)
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2.
3.
4.
5.
Physics
Move the wavelength slider (a) until it reads 350 nm.
The light intensity should still be set to 100%. Drag slider (c) to find the stopping voltage for this
wavelength and light intensity (again when the current is zero).
Keeping the same wavelength, drag slider (b) to the next intensity on Table 2 and again measure the
stopping voltage. Repeat for the remaining intensities on Table 2.
Calculating the electron kinetic energy: As before, the kinetic energy is the same numerical value as the
stopping voltage (without the sign), but the units change from V to eV.
Analysis
Frequency dependence
1.
2.
3.
4.
5.
6.
7.
8.
Using your data from Table 1, enter into Excel the light frequency in the first column and the electron
kinetic energy (eVstop) in the second. Large numerical values are entered using scientific notation:
1.234×1014 is entered by typing 1.234E14.
Create a graph of electron kinetic energy as a function of light frequency. Title your graph and label the
axes.
Add a linear trendline to your graph and choose to display the equation on your graph. Your data points
should follow a straight line very closely. If a point deviates significantly, check your calculations and
make sure you have measured it correctly.
Next we need to make Excel give us more significant figures in our trendline
equation. Right-click the trendline equation and choose Format Trendline
Label to open a pane on the right side of the window, as shown at right.
Under the Category section choose Scientific with 2 decimal places.
Copy your graph and paste it into the end of this document.
Now you have the equation in the form of a line, y = mx+b. In words,
answer the following questions: What does y represent? What is x? What is
m? What is b? Write your answers after your graph.
Compare the value of your slope to Planck’s constant, h = 4.14×10-15 eV•s.
Explain why sometimes the electrons make it from one plate to the other and
sometimes they don’t.
Intensity dependence
1.
2.
3.
4.
Using your data from Table 2, enter into Excel the light intensity % in the fourth column and the electron
kinetic energy (eVstop) in the fifth.
Create a graph of electron kinetic energy as a function of light intensity %. Title your graph and label the
axes.
You will now set the vertical axis so that it starts at zero: Right-click the numbers on the vertical axis and
choose Format Axis. In the pane that appears, under the Bounds section change the Minimum value to 0.
Paste your graph into this document. Describe the shape of your graph.
Discussion
Use your data to answer the following question:
•
Are your data consistent with the particle nature of light or the wave nature of light? Explain and use your
data to support your conclusion.
©SLU Physics
Revised: 4/4/2020
The Photoelectric Effect
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Physics
Running Physics Simulations for Online Labs
This document contains basic information that you’ll need to run some of the simulations
(sims) that we will be using for our ‘remote’ lab experiments. Many of the sims will run in your
favorite web browser (Firefox, Chrome, Safari, Edge) without any additional configuration; they
may even run from your smart phone or tablet, although you might find the screen size a bit too
small. Other sims require the installation of Java so that they can be downloaded and run from
your computer (sorry – these won’t run on phones or tablets). The simulation file name will end
with .jar; double-click this file to start the sim.
Follow the directions below to install the latest version of Java; note that the installer will
look for old versions of Java and offer to remove them. It’s always a good idea to remove any
older versions. Note that you need to be an administrator on your computer to install Java (but
not to run the simulations). You may need to talk to the owner if you’re using a shared computer.
There is also a section below about an errors that Windows or Macintosh users may
encounter when trying to run a Java sim the first time.
Installing Java: These instructions apply to all computer operating systems.
Go to the Java download page: http://java.com/inc/BrowserRedirect1.jsp?locale=en
This page will allow you to download and install the latest version of Java for your
operating system (Windows, Macintosh or Linux). Click the big red button, then follow
the instructions.
You might wish to save the Java installer program. If you would like to uninstall Java in
the future, run the installer again and click the Remove button.
Macintosh Error Message:
When you first open a Java sim, a dialog box (Figure 1) appears
saying the sim “…can’t be opened because it is from an unidentified
developer.”
This is MacOS Gatekeeper, and it is designed to prevent
potentially malicious apps from launching. You can still run
the sim as follows:
o If the warning message is still open, click the OK button
(Figure 1).
o Right click (or hold down the Control key while clicking)
on the sim’s .jar file
o The same warning message will appear as before, but now
there is an Open button (Figure 2). Click Open, enter
your computer password if asked, and the sim will start.
SLU Physics
Revised: 3/23/2020
Figure 1: Error message when
first running a sim
Figure 2: Message box after right‐
clicking the sim file
Department of Physics
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Physics
Windows – An Application Other Than Java Starts:
After you successfully install Java on your computer, you
double-click the .jar simulation file, and something other
than Java starts up (Figure 3). You need to reset the default
application used for .jar files.
Figure 3: An application other than Java
Type Default Apps in the search window (the
starts when starting the sim
magnifying glass) next to the Windows Start menu.
Select Default Apps in the search results. In the Settings window that opens, scroll down
to the bottom and click the link that reads Choose default apps by file type.
In a few moments, a list of file extensions (in the first column, “Name”) and the default
application for that file type. Scroll the list down until you find .jar listed in the first
column.
As shown in Figure 4, the wrong
application is set for .jar files (my
Figure 4: The wrong application (“Cool File Viewer”) is
computer was set to “Cool File Viewer”;
associated with .jar files
your computer may have a different
application chosen).
Click the application name next to .jar and the Choose an
app menu (Figure 5) will pop up with the applications on
your computer that can open this file type. Select
Java(TM) Platform SE binary.
In a few moments you’ll be returned to the default app list,
and Java should be listed as the default app (Figure 6).
Close the Default Apps setting window. You should now
be able to double-click the original .jar file you
Figure 5: Select Java as the correct app
for .jar files
downloaded, and Java should start and run the simulation.
Figure 6: Java will now open .jar files when they are double‐
clicked
SLU Physics
Revised: 3/23/2020
Department of Physics
Canton, NY 13617
ST. LAWRENCE UNIVERSITY
Physics
Graphing and Curve Analysis Using Microsoft Excel
The following instructions guide you through the process of using Microsoft Excel to create a simple x-y scatter
plot (called a “chart” in Excel) from a set of data, and have the program calculate and draw a line that best fits your
data (called a “trendline” in Excel). All students have access to Microsoft Office 365; contact the Information
Technology Helpdesk (315-229-5770) if you need assistance installing Office on your computer.
All functions listed in these instructions can be found by clicking the named tab on the ribbon that appears at
the top of your document window (see Figures 1 and 2 below). The current version of Excel for Windows and
Macintosh are virtually identical, and the functions listed in these directions will work the same for both operating
systems. You’ll note that groups of commands on each ribbon tab are labeled in Excel for Windows, but they can be
turned on in Excel for Macintosh by following the instructions under Figure 2.
ADDITIONAL NOTE FOR MACINTOSH USERS: Several functions are accessed by right-clicking on an
object, which brings up a floating menu of options. If you are not using a two-button mouse, simply press
and hold the control key while you click on the object.
Figure 1: Excel for Windows ribbon: There are tabs labeled File, Home, Insert, Page
Layout, etc. Note that the Home tab is selected, and groups of commands are labeled
(Clipboard, Font, Alignment, Number, etc.)
Figure 2: Excel for Macintosh ribbon: There are tabs labeled Home, Insert, Page Layout,
etc. Note that the Home tab is selected, but groups of commands are not labeled by
default as in the Windows version (Figure 1). Group labels can be displayed by choosing
the Excel menu, then Preferences. Click the View preference, and put a check mark next
to Ribbon Titles in the bottom section (“In Ribbon, Show…”)
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Revised: 3/22/2020
Graphing & Curve Analysis Using Microsoft Excel
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Physics
Creating a Graph
1.
Note that worksheet columns are designated by a letter (A, B, C, etc.) and rows are designated by number.
The first column will contain the values that you will plot along the horizontal (x) axis, the second column
will contain the values to be plotted on the vertical (y) axis. This is true no matter which column you
choose to start entering the data. Excel is very rigid about this layout, so make sure you enter your data
correctly. You should also include column titles in the first row to make sure you enter and plot your data
correctly.
2.
Click one cell (not the column) that contains your data; Excel will
assume the data to be plotted is in the selected and adjacent
columns.
3.
Click the Insert tab on the ribbon. In the Charts group, click the
X Y (Scatter) button, then choose “Scatter” (as highlighted in
Figure 3). A graph will appear in your worksheet.
4.
Add axis labels as follows: with the graph selected, click the Add
Chart Element button (in the Chart Layouts group on the far left),
then Axis Titles and then Primary Horizontal. Repeat these
steps, this time choosing Primary Vertical axis title. Double-click
each “Axis Title” and change it to an appropriate label with units!
5.
Double-click the title region and enter a descriptive title for your
graph. You do not need to include units in the title.
Figure 3: Select the Scatter chart type
Adding the Best-Fit Function
6.
Now that you have created your graph, you need to add a best-fit line, called a trendline in Excel. Rightclick on any data point and choose “Add Trendline…”; a linear fit is applied to your data (even though this
may not be the correct function for your data!) The “Format Trendline” panel appears to the right of your
graph containing six best-fit functions.
Excel will determine which of the six functions can be used for your data (but it doesn’t pick the correct
function for you!) In this course we might use one of these four fits: “Exponential” (y = aebx); “Linear” (y =
mx + b); “Polynomial” (of 2nd order: y = ax2 + bx + c); or “Power” (y = axb).
7.
Select the desired function that best fits your data. If you pick the wrong fit and have closed the “Format
Trendline” window, right-click on the trendline and choose Format Trendline again.
Important note: If you pick a functi…
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