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Concept Questions for the Photoelectric Effect with Interactive Simulation

Terry Bradfield, Department of Natural Science, Northeastern State University, Tahlequah, OK, based on a simulation created by Angel Franco Garcia, Escuela Universitaria de Ingenieria Tecnica Industrial de Eibar
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This material is replicated on a number of sites as part of the SERC Pedagogic Service Project
Initial Publication Date: July 30, 2007


Illustration of Photoelectric Effect
These are interactive lecture-demonstration questions that probe student understanding of fundamental concepts in the photoelectric effect. One question deals with the role of the stopping potential in the experiment. The second relates to the role of wavelength and photon energy. The demonstrations connected to these questions are performed with a Java Applet simulation of the photoelectric experiment. The simulation provides a direct visualization of the process that is not available in the actual experiment.

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Learning Goals

The questions probe common misconceptions about the role of the stopping potential and photon energy in the photoelectric experiment that measures the electrons' kinetic energy. The simulation is then used to confront them with results that contradict predictions based on these misconceptions. The learning goals for this activity are:
  1. Students should understand the relationship between photon wavelength and energy and its relation to the kinetic energy of the photoelectrons.
  2. They should also understand how the stopping potential is used to determine the electrons' kinetic energy.

Context for Use

These questions are to be used for in-class peer instruction. Students should make, share and discuss their predictions prior to the demonstration. There should also be a general class discussion/presentation afterward to clarify the material. Students should have had an introduction to the idea of photons and the relation between photon energy and frequency.

Description and Teaching Materials

The simulation is embedded within a Spanish Language Page by Angel Franco Garcia. It can be run directly from

Question 1: Effect of Changing the Potential (Microsoft Word 35kB Jul30 07)
Question 2: Effect of Changing Wavelength (Microsoft Word 35kB Jul30 07)

Teaching Notes and Tips

Using the Simulation

Before presenting the question, students should be allowed to see the simulation operate. They should also be given a minimal verbal description of the process. An example: The battery in this system pulls some electrons from the plate on the left and places them on the other plate. The plates now have equal and opposite charges, and there is an electric field in the region between them that points to the right. When the Foton (photon) button is pressed, the Lampara (lamp) emits a photon toward the left plate, where it is absorbed by an electron that is then ejected into the region between the plates. Since the captions are in Spanish, extra care is needed to be certain students are clear on the names and functions of any components in the simulation you are using.

Notes about Question 1

B is the correct choice. Increasing the potential difference increases the field strength which results in a larger retarding force on the electron.

Students who choose A may think that the potential is causing the electron to move across the gap rather than opposing it. Those choosing C may not realize that the electric field strength is determined by the potential difference.

Notes about Question 2

A is the correct choice. Decreasing wavelength increases frequency so the photon imparts more kinetic energy to the electric.

Students who choose B may be confused by the inverse relationship between energy and wavelength as compared to energy and frequency. Those who choose C may not realize that the source of the electron's energy is the photon it absorbs.


Qualitative questions of a similar nature can be included on exams.

References and Resources

For additional information about student's difficulties with the photoelectric effect, see Five Easy Lessons, Strategies for Successful Physics Teaching by Randall D. Knight, Addison-Wesley ISBN 0-8053-8702-1