Beacon Lesson Plan Library

Light on Trial: Wave or Particle?

Robert Rosen


Students determine whether light is a wave, a particle, or some combination of each by presenting evidence in a mock trial format.


The student selects and uses appropriate listening strategies according to the intended purpose (such as solving problems, interpreting and evaluating the techniques and intent of a presentation, and taking action in career-related situations).

The student uses effective strategies for informal and formal discussions, including listening actively and reflectively, connecting to and building on the ideas of a previous speaker, and respecting the viewpoints of others.

The student develops and sustains a line of argument and provides appropriate support.

The student understands that matter may act as a wave, a particle, or something else entirely different with its own characteristic behavior.

Demonstrates ability to gather information from various sources to plan a project.

Analyzes the managerial skills necessary for decision making in different work related situations.

Creates potential solutions to industry problems using math and/or scientific concepts and communicates solution using industry appropriate language arts and graphic skills.


-Reference materials should include college and university level physics textbooks and on-line data bases
-Poster board and markers for producing diagrams
-Equipment for demonstration of wave-like properties
-Ripple tank
-Camera film
-Soap bubble solution
-Copies of the rubric found in the attached file


1. The instructor should introduce the general properties of all waves: reflection, refraction, rectilinear propagation, diffraction and interference.
This lesson will provide students with useful background information.

2. The instructor must decide how much time to devote to this activity. The more open-ended the assignment, the more class time will be required for research and preparation. If the instructor chooses to give a minimum number of hints, students will need time in the library to search physics references and on-line sources to learn about light and then to learn about specific phenomena that will bolster their case. Time needed to prepare for the trial could be shortened if the instructor lists some phenomena for students and provides appropriate physics textbooks. Relevant phenomena for the defense include the following;
refraction, polarization, diffraction, single slit interference, double slit interference and interference in thin films. Relevant phenomena for the prosecution include the photoelectric effect, spectral lines produced in a gas discharge tube, blackbody radiation laws, and Compton scattering.

3. If a more close-ended approach is desired, students can be directed to study de Broglie wavelength, electron diffraction (Davisson-Germer experiment) and the quantum hypothesis. Twentieth century researchers believe that symmetry in the universe is expressed in the wave-particle duality. That is, electromagnetic waves have particle-like
properties and particles have wave-like properties. Electromagnetic waves and objects such as electrons are complex and do not fit either a simple pure wave or particle model. A background in modern physics will enable the jury
to make sense out of the apparently contradictory evidence they will hear. In addition, when the jury explains its verdict, members of the prosecution and defense teams will learn from the jury how modern physics resolves the
wave-particle paradox.


-Students know the historical development behind the wave-partical theories of light.
-Students demonstrate characteristics of a wave and characteristics of a particle.
-Students understand common arguments behind the explanation of wave-partical duality.
-Students present a clear explanation behind the behavior of light.
-Students know some of the main scientific contributions to the development of the theory.
-Students show, through calculations, relationships of energy, frequency and wavelength.
-Students show, through calculations, the relationships of mass, frequency and wavelength.
-Students understand the structure of the electromagnetic spectrum and be able to identify its components.

1. Inform students that they will be participating in a trial to determine if light is a wave, a particle, or some combination of each.

2. Divide students into two groups. One group will become the prosecution; the other will become the defense.

3. Assign the following tasks:
(a) Prosecution: Through research and demonstration, you will attempt to prove light is a particle.
(b) Defense: Through research and demonstration, you will attempt to prove light is a wave.

4. Allow each group to gather information, prepare demonstrations, pictures, posters, film clips, or any other material that they may wish to use in preparation of their argument.

5. Inform the students that after gathering information, members of the defense and prosecution team will be given class time to synthesize and organize their cases. Instruct each team to plan the way their research should be presented and to make their case as convincing as possible.

6. On the day of the trial, six students will be selected from each group. These twelve students will represent the jury, and they will have the task of deciding the verdict, and explaining their verdict to the class.

Allow each team to present its case to the jury. Unlike a real trial, the defense and prosecution will not be questioning witnesses. However, each team member should be prepared to answer questions based on his contribution. Questions should be posed by the instructor, and the jury should be encouraged to ask questions as well.

7. At the end of the two presentations, administer a quiz to the prosecution and defense based on highlights of the presentations. The questions can be given to help assess each studentís understanding of the presentations. While the defense and prosecution teams are taking the quiz, the jury can deliberate. The jury should work as a team to organize their response. Their comments should clarify the wave-particle nature of light for the class.


Assess the presentations using the rubric in the associated file.

1. Students know the historical development behind the wave-particle theories of light.
2. Students demonstrate characteristics of a wave and characteristics of a particle.
3. Students understand common arguments behind the explanation of wave-particle duality.
4. Students present a clear explanation of the behavior of light.
5. Students show, through calculations, relationships of energy, frequency, and wavelength.
6. Students show, through calculations, the relationships of mass, frequency, and wavelength.
7. Students understand the structure of the electromagnetic spectrum and be able to identify its components.

Questions may include the following:

1. From our discussion, it is clear that:
a. Light is a wave.
b. Light is a particle.
c. Light is bothn a and b.
d. Light is neither a or b.

(answer c. Light is a wave and a particle)

2. Which true statement helps explain the dual nature of light?
a. Particles travel in waves, and have wave properties.
b. Waves do not have particle properties.
c. Particles do not have wave properties.
d. Waves and particles are not related.

( answer a. According to DeBroglie, particles travel in waves, and the nature of the wave is dependent upon the mass of the particle)

3. Which relationship is true about wave nature?
a. Frequency and wavelength are not related.
b. Wavelength and energy are not related.
c. Energy increases as wavelength increases.
d. Energy increases as frequency increases.

(answer d. Energy increases as the frequency increases which decreases the wavelength)

The following summary is provided to help instructors assess student presentations. Topics that should be covered in a thorough student presentation are briefly described.

Spectral Lines Produced by a Gas
When an electrical spark passes through gas in a discharge tube, the gas gives off light only at certain frequencies that depend on the gas. The spectrum of the light thus consists of a series of lines. The spectrum is not continuous like the solar spectrum. The spectral lines can be explained by assuming that atoms have discrete energy levels- only certain energies are permissible. An atomís energy can change from one of the permitted values to another. When an atomís energy changes, a photon whose energy equals the difference between the permitted energy levels is given off (or absorbed). Spectral lines can be understood by using a photon model of light (light energy comes in particle-like packages, not a continuous stream). Spectral lines can not be explained using a wave model of light.

Blackbody Radiation Laws
Hot objects radiate light. A blackbody is an idealized radiator that scientists use to predict the nature of the radiated light. The amount of light energy radiated at each frequency can be predicted if the temperature of the radiator is known. From this energy distribution, the total amount of energy emitted (Stefan-Boltzmann law) and the frequency corresponding to the maximum energy (Wien displacement law) can be calculated. But these energy-frequency relationships can be derived only by assuming a photon (particle) model for light.

Compton Scattering
When X-rays strike an electron, the electron recoils and the reflected X-ray has a longer wavelength (lower frequency) than the original X-ray. According to the wave model, the frequency of the reflected X-ray must be the same as the frequency of the original X-ray. So the wave model can not explain the scattering of an X-ray by an electron. But if you apply the same conservation of energy and conservation of momentum laws we use to study the collision of billiard balls (particles) then you can understand what happens when an X-ray (photon) collides with an electron. Thus, a particle model of electromagnetic radiation can be used to understand Compton scattering.

Reflection is a property of light that can be understood using either a wave or a particle model. Therefore, studying the reflection of light does not help us decide whether light is a wave or a particle.

Student Reflection on Learning:

Physicists feel that symmetry is one of the most important properties of the Universe. Have students write an essay describing some of the symmetries of the Universe. The essay must include the symmetry they encountered in this activity.


-Invent a new word which may better explain the true nature of light.
-Present this to other classes for input and development.

Attached Files

A rubric to assess the presentations.     File Extension: pdf

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