This is the teacher guide for this lesson. A student-focused guide to assist learners as they perform the activity is available.
Teacher Guide
Particle-Wave Duality
What models best explain the behavior of light? Does light behave like a wave, a particle, neither, or both?
What models best explain the behavior of light? Does light behave like a wave, a particle, neither, or both?
- Laser pointer, any kind (red lasers work best because of its larger wavelength)
- Thin copper filament/strand. Many wires are made with copper strands. Cut a piece of a cable that you don’t mind discarding. Remove the rubber and separate the intertwined pieces of copper. Get a single strand, depending on your laser pointer 3 inches will suffice.
- Electrical tape
- Paper, notebook or whiteboard to write the observations
- Note: You can find other DIY setups that use old DVD discs, aluminum foil, or even cardboard paper. We have found this the easiest and most reliable form of doing it.
- A demonstration can be found here: https://www.youtube.com/watch?v=kKdaRJ3vAmA
Scientists use experiments to develop an understanding of the world around us. We propose hypotheses and design experiments to test them or we ask questions like “why do objects/things always fall to the ground?”. Often the technology that we can use to do our experiments changes and evolves, which allows us to refine our experiments and therefore expand our knowledge and understanding of the world. What is light? Have you ever wondered how light travels, whether it moven a straight line or if it spreads in multiple ways? Two of the questions scientists considered in the early 1700s were “what is light?” and “how does it travel?” For many years, there were different theories, and physicists designed many experiments to test the different hypotheses. Today you are going to follow in their footsteps to answer these questions yourself.
Safety
- Do not point the laser at anyone, and do not look directly into the laser.
- Total time45 - 60 Minutes
- Education levelGrades 6 - 10
- Content AreaQuantum Mechanics
- Educational topicLight, Waves, Quantum Science
The concept behind how light travels and behaves has been one of physics’ greatest mysteries. In the world we experience every day, we see that objects, like a chair or a rock, can only be in one place at one time. We can say that these objects behave like a particle, which is a tiny object that is characterized by only being in one place at a time. On the other hand, we have things that can be at different places at the same time, such as waves. Have you noticed what happens when you throw a rock in the water? It makes waves that spread in a ring and grow as they move outward. In physics, waves are described as the spread of the disturbance or perturbation of something, often energy. When you throw a rock in the water, you are transferring some energy of the movement of the rock into the water, which causes the water to move around to disperse that energy.
We can see how a rock behaves differently from a wave. But scientists could not agree if light behaves like a particle or a wave In 1801, Thomas Young, a British scientist, was convinced that light was a wave, and he designed his famous double-slit experiment. In this experiment, Young would shine a light source onto a screen that had two small slits close to one another to see the reflection of the light after passing through the slits in the wall behind it. What he saw was what physicists now call an interference pattern. Let’s go back to the ripples in the water. If we throw a rock in the water, we see that it creates waves that have crests and troughs. If we throw a rock next to it, we will see that it also creates crests and troughs. As the waves spread, we will see that the waves that originated from one rock will start to cross the path of the waves created by the other rock. When the crests of waves created by rock A cross the troughs of waves created by rock B, they will cancel each other out. When the crest of waves created by rock A crosses the crest of waves created by rock B, then they form a taller crest. Then you can imagine what happens when the paths of two troughs cross each other: they forma deeper trough.
If instead of thinking of water waves now we think about light, then what we expect to see on the wall will be lines of bright light and lines with no light. That is exactly what Young’s double-slit experiment showed. Confirming the hypothesis that light behaves as a wave. So then, that’s it, right? Light is a wave! Yes, but…
In order to be sure, other scientists decided to explore the subject a bit further. In 1909, Geofrey Taylor repeated Young’s experiment and took photos of the light coming out of the slits. Initially he saw the same interference patter as Young. But when he dimmed the light to very low level, the photos were of little points, like particles hitting the screen. If he left exposure of the photo for long enough, the little points started forming the interference pattern. Therefore, he demonstrated that light can behave both as particle and wave. Many other scientists throughout history have tested this concept with electrons, molecules, and atoms always seeing the same result. Now it is time for you to check it out!
Key terms
These are the key terms that students should know by the END of the two lessons. They do not need to be front loaded. In fact, studies show that presenting key terms to students before the lesson may not be as effective as having students observe and witness the phenomenon the key terms illustrate beforehand and learn the formalized words afterwards. For this reason, we recommend allowing students to grapple with the experiments without knowing these words and then exposing them to the formalized definitions afterwards in the context of what they learned.
However, if these words are helpful for students on an IEP, ELL students, or anyone else that may need more support, please use at your discretion.
- Light: Electromagnetic radiation of any wavelength, whether visible or not.
- Energy: Quantitative property that must be transferred to a body or physical system to perform work on the body, or to heat it.
- Wave: A physical phenomenon characterized by its frequency, wavelength, and amplitude which transfer energy from one location to another.
- Particle: A measurable change in momentum involving a matter wave interacting with something.
- Quantum Mechanics: The branch of mechanics that deals with the mathematical description of the motion and interaction of subatomic particles, incorporating the concepts of quantization of energy, wave-particle duality, the uncertainty principle, and the correspondence principle.
Objective
- Student objective
*Students will develop an experiment, following Young’s experiment, to see if they can determine whether light behaves as a particle or a wave. The idea of this experiment is that students start to form a model of the light’s behavior as a particle and as a wave.
*It is important to understand that student goals may be different and unique from the lesson goals. We recommend leaving room for students to set their own goals for each activity.
Before the Experiment
- Give student groups a white board
Give student groups a white board. Ask them the following questions: What do they expect to see when a particle goes through an obstacle such as small holes in a paper? What would they expect to see if a wave when through a piece of paper? What do they expect light would look like? Ask them to draw their models.
- Group students together
- Give them a minute to think quietly
- Give students 2 minutes to discuss their thinking
- Have students record their answers on the white boards and share with the class
Setting Up
- Let students carefully shine the laser
Once predictions are complete, let students carefully shine the laser onto a wall or a screen. Ask them to write down their observations and how what they saw compares to models of particles (thinking about sand grains as particles) and waves without obstacles.
- Ask students to build their double slit diffraction set up
Next, ask students to build their double slit diffraction set up.
- Take the piece of copper filament/strand and place it across the laser pointer, where the beam comes out.
- Use electrical tape to secure the copper strand to the laser pointer on the top and bottom. Then add extra tape to the side to make a smaller aperture for the beam to go through.
During the Experiment
- Shine the laser light onto the screen or wall.
Shine the laser light onto the screen or wall. You should see a horizontal pattern of dark and bright lines or dots that represent the crests and troughs of the interference of the light as it passes through the double slits. Patterns are easier to see if the lights are off and the wall or screen is flat with no edges that interfere with the light as it passes through the double slit.
WARNING: Be careful to not shine the laser on people’s faces or eyes.
- Examples of what you should see on the screens/wall
Figures below are two examples of what you should see on the screens/wall. The top picture patterns were created with the laser pointer set up shown above. The bottom picture shows a double slit pattern from a physics lab.
- Draw what they see and compare to their predictions
Draw what they see and compare to their predictions and also to what they expect particles or waves will be like.
- This double slit interference pattern was recorded in a laboratory
This double slit interference pattern was recorded in a laboratory. (credit: PASCO)
Teacher tip
- Suggested STEP UP Everyday Actions to incorporate into activity:
- When pairing students, try to have male/female partners and invite female students to share their ideas first.
- As you put students into groups, consider having female or minority students take the leadership role.
- Take note of female participation. If they seem to be taking direction and following along, elevate their voice by asking them a question about their experiment.
- Consider using white boards so students have time to work through their ideas and brainstorms before saying them out loud.
- As students experiment, roam around the room to listen in on discussion and notice experiment techniques. If needed, stop the class and call over to a certain group that has hit on an important concept.
Consider using the RIP protocol (Research, Instruct, Plan) for lab group visits and conferring.
Consider culturally responsive tools and strategies and/or open ended reflection questions to help push student thinking, evidence tracking, and connections to their lives.
Conclusion
- How do your observations explain the need for a theory of wave-particle duality of light?
How do your observations explain the need for a theory of wave-particle duality of light?
- Real world connections -
- Have students watch this video to dive deeper into how Einstein applied the wave-particle duality theory
- Have students explore these resources and activities from the Wonders of Physics
- Suggestions for drawing, illustrating, presenting content in creative ways
- Have students play with this Phet simulation to visualize this experiment. Have them write about how the visualization helped them understand the concept more deeply.
- Engineering and design challenges connected to the content
Tape Art is a fun and engaging way to introduce students to some of the basic properties of light. This activity uses transparencies, tape, and polarizers to help students see how they affect the color of light seen.
- MS-PS4-1-empirical-evidenceSEPs: Scientific Knowledge is Based on Empirical Evidence. Science knowledge is based upon logical and conceptual connections between evidence and explanations. (MS-PS4-1)
Credits
Coordination, Research, Text, and Editorial Review Jessica Eskew, Jamie Liu, Leah Poffenberger, Catherine Tabor, Laurie Tangren, and Rose Villatoro
Graphic Design and Production Meghan White
Cover Illustration Annamaria Ward
Activity 3 Save Schrodinger’s Cat is created by the team behind Quarks Interactive SRL. Concept: Laurentiu Nita
Design & Art: Ar. Judit Balazs-Becsi
Writing: Dr. Nicholas Chancellor, Dr. Helen Cramman, Dr. Laura Mazolli Smith, Andrei Voicu Tomut
Activity 4 Quantum Circuits was created by our partners at Virginia Tech Sophia Economou and Edwin Barnes. Supported by the National Science Foundation (grant nos. 1741656 and 1847078)
PhysicsQuest is sponsored in part by the Eucalyptus Foundation.
Updated in 2023 by Sierra Crandell, M.Ed. partially funded by Eucalyptus Foundation
Extension by Jenna Tempkin with Society of Physics Students (SPS)