Teacher Guide

# Multiplexing Messages

## Experimenting with wave properties of light

How does using different wavelengths of light allow multiple messages to be sent in a single channel at the same time?

This resource was originally published in PhysicsQuest 2023: Making Waves.

This is the teacher guide for this lesson. A student-focused guide to assist learners as they perform the activity is available.

View the student guide: Multiplexing Messages

How does using different wavelengths of light allow multiple messages to be sent in a single channel at the same time?

• Red/cyan 3D glasses
• Markers, highlighters, or colored pencils tuned to wavelengths transmitted and absorbed by glasses
• Paper
• Seven-segment display symbol samples

Students start by exploring with the 3D glasses and markers/colored pencils/crayons to find colors that work well for this activity. It’s important to use markers, highlighters, or colored pencils that are tuned to the wavelength of light that each filter transmits.

The right color will “disappear” when viewed through the 3D glasses. Then students can create “multiplexed messages” — making overlapping letters/numbers with different colors allows you to transmit two different messages on top of each other; you can see one message with the red lens in the 3D glasses and the other message with the cyan lens. Then students are asked to review what they’ve learned so far and think about how they could separate laser light multiplexed messages in a fiber optic cable.

• Total time
30 - 45 minutes
• Education level
• Content Area
Waves
• Educational topic
Waves, features of waves, wave terminology

Watch this video from Little Shop of Physics for an overview of the experimental setup and the science behind the phenomenon.

Students will be introduced to how multiplexing works and combine what they’ve learned about waves to make an educated guess as to how multiplexed signals are eventually separated out.

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.

Key terms

These are the key terms that students should know by the end of the lesson. 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 afterward. For this reason, we recommend allowing students to grapple with the experiments without knowing these words and then exposing them to the formalized definitions afterward in the context of what they learned.

However, if these words are helpful for students on an IEP, ELL students, or anyone else who may need more support, please use at your discretion.

• Multiplexing: Multiplexing is a method used by networks to consolidate multiple signals — digital or analog — into a single composite signal that is transported over a common medium, such as a fiber optic cable or radio wave.
Objective

Students will be introduced to how multiplexing works and combine what they’ve learned about waves to make an educated guess as to how multiplexed signals are eventually separated out.

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
• Watch this video from Little Shop of Physics for an overview of the experimental setup and the science behind the phenomenon.

• Fiber optic internet is a light signal sent through a fiber optic cable.

If you and your neighbors all get internet at your respective homes, and all those internet signals are sent through a single fiber optic cable, how do those signals not get mixed up? How do you get your funny cat videos and your neighbor gets DIY shoelace videos?

1. Students begin in pairs, responding to a discussion question only with a single partner.
2. After each person has had a chance to share their ideas, the pair joins another pair, creating a group of four.
3. Pairs share their ideas with the pair they just joined.
4. Groups of four join together to form groups of eight, and so on, until the whole class is joined up in one large discussion.
• Tell students that the experiment they are about to do will help them explore this idea of multiplexing messages — layering messages in a single channel and separating them out later.

Setting up
• Ask your students where they’ve seen seven-segment displays before. (Older calculators and digital clocks are common places to find them.)

• You’ll need paper, red and blue colored pencils/markers/crayons, and 3D glasses to hand out to the students.

During the experiment
• Students make overlapping letters/numbers with different colors allowing them to transmit two different messages on the same piece of paper; you can see one message with the red lens in the 3D glasses, and the other message with the cyan lens. It’s important for them to find markers, highlighters, or colored pencils that are tuned to the wavelength of light that each filter transmits.

Teacher tip

A great way to start any physics-related unit is with the STEP UP Careers in Physics lesson. This lesson covers careers one can do with a physics degree, particularly those that help solve societal problems. It helps students assess their personal values in relation to a career in physics, examine profiles of professionals with physics degrees, and envision themselves in a physics career.

Suggested STEP UP Everyday Actions to incorporate into the 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 females or students from underrepresented backgrounds take the leadership role.
• Take note of female participation. If they seem to be only receiving direction and following along, elevate their voice by asking them a question about their experiment.

Consider using whiteboards so students have time to work through their ideas and brainstorm 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
• After experiencing how the 3D glasses together with the right colored pencils can produce overlapping messages that can be viewed separately, students should answer the question:

How does multiplexing allow you to use a single channel for multiple messages simultaneously?

1. Divide students into groups of four.
2. The teacher posts a question that students must answer with an explanation.
3. Students each write their own ideas on a loose piece of paper.
4. Then the papers are all passed to the left.
5. Each student silently reads the previous student’s response (and any of the other students’ comments, on iterating rounds of this process).
6. Each student writes suggestions directly onto the original copy to help make their peers’ ideas sharper and clearer.
7. Repeat the pass-read-edit process until each student gets to read and comment on each others’ ideas.
8. The original author of each statement reads their peers' comments and writes a refined, final statement at the bottom of the paper to turn in.
• Introduce the PhysicsQuest 23: Making Waves Physics Career and Concept Map and allow students to read through and discuss the careers that use this content. Extend their thinking with research about these careers if time allows.

• Real-world connections:
• A lot of people own household phones and cell phones. Using what you learned, explain how multiple phones use the same cell phone tower.
• Suggestions for drawing, illustrating, and presenting content in creative ways:
• Gather up your whole class for a 3D group photo!
• Using the free app called “MakeIt3D - 3D Camera” you can take two pictures of your class (have everyone stand perfectly still!), the app will overlay them, and the end result really pops! Having students extend their arms toward or away from the camera gives extra depth!
• Create a scavenger hunt/escape room for students using blue and red colors and 3D glasses. Have one team look for clues in blue text and the other team look for clues in the red text.
• Build a seven-segment display.
• Engineering and design challenges connected to the content:
• Imagine you are stranded on a deserted island. How could you use the properties of multiplex messaging and/or light to try and be rescued? Be creative!
• If engineering challenges have a time constraint, students are allowed to keep iterating and developing their ideas outside of class time and continue to participate in the challenge at a later date.
• Watch this Listen to Light video and see what lasers sound like!

### Women in physics

This lesson introduces the underrepresentation of women in physics and the role of implicit bias and cultural stereotypes. Helps students examine the conditions for women in physics and helps students discuss gender issues, gendered professions, and personal experiences to neutralize the effect of stereotypes and bias.

• MS-PS4-2
Develop and use a model to describe how waves are reflected, absorbed, or transmitted through various materials.

### Credits

Created by Cherie Bornhorst, MEd, and Little Shop of Physics along with Nicole Schrode, MEd, and Claudia Fracchiolla, PhD, of APS Public Engagement

Reviewed by Summer Chrisman, MEd, Tamia Williams, MSt, Chris Irwin

Extensions by Jenna Tempkin

Formatted by Sierra Crandell, MEd, partially funded by Eucalyptus Foundation