# Quantum Mechanics Lesson 1: Wave Properties of Light

## Quantum Mechanics

Kevin Warren, Laura Halstied | Published: August 17th, 2023 by K20 Center

• Subject Science
• Course Physics

### Summary

In this lesson, students first review the principle of superposition of waves and constructive and destructive interference with slinkies. Students then investigate diffraction and how different variables change the interference pattern in the double slit experiment using the Wave Interference Phet Simulation. Next students develop an intuitive understanding of why changing the wavelength of light, the distance to the screen, and the distance between the slits will change the distance between the bands in the interference pattern in the double slit experiment. Lastly, students predict how the Classical Wave Theory applies to the photoelectric effect, and they will be shown the results from the experiment that contradict the Classical Wave Theory to show the need for a new theory.

### Essential Question(s)

Is light a wave or a particle?

### Snapshot

Engage

Explore

Students simulate waves to analyze constructive and destructive interference through a Phet Simulation.

Explain

Students read about constructive and destructive interference, then reflect in small groups.

Extend

Students build consensus on questions about Classical Wave Theory.

Evaluate

Students demonstrate their understanding of constructive and destructive interference by answering questions after observing several colored lasers.

### Materials

• Lesson Slides (attached)

• Wave Interactions Teacher Guide (attached)

• Behavior of Light as a Wave handout (attached, 1 per student)

• Behavior of Light as a Wave Teacher Guide (attached)

• Diffraction and Double Slit Experiment handout (attached, 1 per pair of students)

• Diffraction and Double Slit Experiment Teacher Guide (attached)

• Classical Wave Theory Teacher Guide (attached)

• Exit Ticket Teacher Guide (attached)

• Extra long slinky (1 per group of 3-4 students)

• Masking Tape (1 roll per group of 3-4 students)

• Meter Stick (1 roll per group of 3-4 students)

• Two different colors of laser lights (1 set for teacher demonstration)

• Two different sizes of diffraction grating (1 set for teacher demonstration)

• Student devices with internet access (1 per group of 2 students)

• Notebook paper

• Pencils

### Engage

40 Minute(s)

Use the attached Lesson Slides to guide the lesson. Display slides 1-4 to introduce the essential questions and learning objectives with students.

Move to slide 5 and divide the students into groups of three. Each group will need an extra-long slinky, masking tape, a meter stick, a piece of notebook paper, and a pencil to set up their station on the floor and record their group's answers. Display slides 6-8 to have the students create, record, and hypothesize about constructive interference. Display slides 9-11 to have the students create, record, and hypothesize about destructive interference.

Keep the students in the same three-person groups and display slide 12 to have the students predict the answer on their group paper. Have two or three groups explain their answers to gain class consensus before moving on. Repeat the same process with slide 13. Display slide 14 and have each group discuss their answer and record it on their group paper. Have two or three groups explain their answer to gain class consensus before displaying the answer on slide 15. Do the same process with slides 16-17 for defining destructive interference.

Display slide 18 to have the groups predict the answer on their group paper. Have 2 or 3 groups explain their answers to gain class consensus before moving on. Repeat the same process with slide 19. Display slide 20 to review the concept of the superposition of waves. Display slide 21 and have the students use the Point of Most Significance instructional strategy to have students reflect on what was learned.

### Explore

55 Minute(s)

Pass out the attached Diffraction and Double Slit Experiment handout and have students break into groups of two with a laptop. Tell students to access the activity at https://phet.colorado.edu/en/simulations/wave-interference. The website is also on the handout for students. Display slide 22-24 to help guide them to part 1 of the simulation and have each person fill out their handout as the group spends 20 minutes to work through part 1 of the handout. Display slide 25-27 to help guide them to part two of the simulation and have each person fill out their handout as the group spends 25 minutes to work through part 2 of the handout.

Display slides 28-29 to use the modified What did I Learn Today? instructional strategy to have the students spend ten minutes individually answering the two questions to show where they are at this point in the lesson. See the attached Diffraction and Double Slit Experiment Teacher Guide for sample student responses.

### Explain

40 Minute(s)

Pass out the attached Behavior of Light as a Wave handout and have the students read it individually before answering the questions on slide 30 on a separate sheet of paper in small groups.

Have two to three groups share their responses.  Go over the I Notice / I Wonder instructional strategy and then show the videos on slides 31-32. Have the students use their I Notice/ I Wonder table to go back and reevaluate their answers to the questions on slide 30 before turning them in. See the attached Behavior of Light as a Wave Teacher Guide for sample student responses.

### Extend

55 Minute(s)

Display slides 33-34. Read over each of the slides and explain that they will be using the information on these slides about Classical Wave Theory to justify their answers to questions on the following slides. Have the students take notes and ask questions about the slides before moving on.

Display slide 35 to explain the modified 8-Up instructional strategy to build consensus in the class that will be used for the four questions on slide 36-39. Give students two minutes to come up with their own answer and justification to the first question. Give students 1-2 minutes to combine with another person to choose one answer and justification for the group of two. Give 1-2 minutes to combine with another group to choose one answer and justification for the group of 4. Give 1-2 minutes to combine with another group to choose one answer and justification for the group of 8. Finish question one with a class discussion where each group of eight shares their response and the class has a discussion to reach consensus. Do the process again for questions two to four.

Display slide 40 and read over the actual results of the photoelectric effect. It is important that the class came to the correct class consensus for the four questions so that they understand that the Classical Wave Theory contradicts the results of the photoelectric effect so that they understand that the results of the experiment showed that the theory needed to be changed, which is where the discussion will be picked up in the next lesson. It might be necessary to contribute your own justification to one of the questions if none of the groups goes in that direction to make sure that they understand how a wave would act and to ensure that they are sufficiently shocked when the actual results are discussed. See the attached Classical Wave Theory Teacher Guide for sample student responses.

### Evaluate

Display slide 41 and explain that students will use the Bell Ringers and Exit Ticket instructional strategy to evaluate student understanding for the unit by having them answer the following questions on their own paper.

Display slides 42-44 for question one shown in red, green, and blue light. Display slide 45 for question two on the effect of the wavelength of the light. Display slides 46-48 for question 3 on the effect of the distance between the slits in red, green, and blue light. Display slide 49 and play the video on Arago’s Spot. Have students turn in their paper after answering the questions. See the attached Exit Ticket Teacher Guide for sample student responses.