Hands-on Activity Exploring Light With Holographic Chocolate

Learning Objectives

After this activity, students should be able to:

• Explain that light travels in a straight line until it strikes an object and is reflected or travels through one medium to another and is refracted.
• Describe light as a spectrum that includes visible and invisible light.
• Summarize the uses of energy, including mechanical, light, thermal, electrical, and sound energy.
• Apply the use of electromagnetic waves in applications by creating a final product that harnesses light energy.
• Explain the differences and similarities between diffraction and refraction.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

International Technology and Engineering Educators Association - Technology

Materials List

Chocolate Tempering Method

Each group needs:

• 600 g chocolate
• a diffraction grating – 12” x 6” (Note: This grating can be reused.) 
• a hot plate
• a pot (double boiler)
• a digital thermometer
• parchment paper
• an aluminum baking sheet
• a paper plate
• a marker
• a hand towel, potholder, or trivet
• a rubber spatula
• a flashlight
• Holographic Chocolate Worksheet

Each student needs:

• science notebook
• pencil or pen
• lab gloves
• lab goggles
• KWL Chart

For the whole class to share:

• laptop and projector to show videos
• access to a refrigerator
• various recyclable materials for the student products
• various adhesives for student products (i.e., clear tape, masking tape, glue, hot glue w/ hot glue gun, etc.)
• scissors

Worksheets and Attachments

Pre-Req Knowledge

Students need a basic understanding of energy and how light behaves (i.e., light refracts/diffracts, reflects, or is absorbed). They need knowledge of how to work in cooperative groups, and knowledge of basic lab safety.

Introduction/Motivation

Today we are going to talk about alternatives to fossil fuels, in particular solar power. First, we will look at how the human consumption of fossil fuels has led to climate change by watching a video. I’d like you to draw a T-chart in your notebook and label the first column “What I Notice,” and the second column “What I Wonder.” While you watch this video, you will add information to your T-chart. Afterward, you will share these thoughts with your small groups and then we will follow up with a whole-class discussion.
 
(Show this video to the students: https://youtu.be/JCFJlCMGmgY (3:11 minutes).)

(Give students five minutes to discuss their thoughts with their small groups or shoulder partners. After five minutes, pull the students’ attention back to the whole class.)

What is causing climate change and global warming? (Let students offer answers.) How can we reduce our dependence on fossil fuels? (Let students offer answers. Guide students toward solar power.)

Solar power comes from the sun. The energy from the sun (i.e., sunlight) is an alternative to fossil fuels because sunlight is abundant and renewable. How can sunlight be used as a replacement for forms of energy that require fossil fuels? (Let students offer answers. Answer: Light energy is used in many applications to create electricity.) Solar panels are a great example of light energy being used as a replacement for fossil fuels.

How do solar panels work? The simplest explanation is this: When the sun shines onto a solar panel, the sunlight is absorbed by cells in the panel. This collected energy transforms into electrical energy (or electrical charges).

A solar panel is made of multiple solar cells called photovoltaic (PV) cells. When sunlight shines on the PV cells, that light may be reflected or absorbed, or pass right through the cell. The PV cell is composed of semiconductor material, meaning that it can conduct electricity better than an insulator but not as well as a good conductor like a metal. When the semiconductor is exposed to light, it absorbs the light’s energy and transfers it to negatively charged particles in the material called electrons. This extra energy allows the electrons to flow through the material as an electrical current. This current is extracted through conductive metal contacts—the grid-like lines on a solar cell—and can then be used to power houses and other things on the electric grid.

The amount of electricity produced from PV cells depends on the characteristics of the light (such as intensity and wavelengths) and the attributes of the PV cell. An important property of PV semiconductors is the bandgap, which indicates what wavelengths of light the material can absorb and convert to electrical energy. If the semiconductor’s bandgap matches the wavelengths of light shining on the PV cell, then that cell can efficiently make use of all the available energy.

To explore how light interacts with matter, we are first going to make holographic chocolate and then you will work in pairs to create your own light device!

Procedure

Background

Solar cells contain semiconductors that conduct electricity when there is sunlight. In sunlight, the semiconductor absorbs light and transfers energy to negatively charged particles (i.e., electrons). Electrons flow through the semiconductor as an electrical current; it travels to an inverter, which converts the direct current (DC) to alternating current (AC) that flows into the electric grid. The energy in the grid can be used to power people’s homes. As long as the semiconductor is hit by the sun, it continues to generate electricity.

Everything we see is based on light interacting with objects in different ways. It can bounce off of, diffract around or bend around objects, or refract, which means change direction. These different interactions are perceived and processed differently by our eyes and brains. Light can also behave as a wave or a particle, depending on how you are observing it. An example of such waves would be radio waves, and an example of such particles would be photons. Photons carry an amount of energy. This is the energy the kids used on their products. Unlike soundwaves, light waves require no medium to travel through and can travel very quickly. Nothing moves faster than the speed of light.

How/why does light have a spectrum? The spectrum is a way to measure different waves of light in nanometers. Light is a form of energy, and the spectrum is used to determine how much energy light has. Long wavelengths correspond with low energies, and short wavelengths correspond with high energies.

What are visible light and invisible light? How are they related to the light spectrum? Visible light is the light we can see. This is ROYGBIV, or the colors of the rainbow. Invisible light is gamma rays, x-rays, ultra-violet (UV), infrared, and radio waves. They all fall on the spectrum, with visible light falling between UV and infrared. Radio waves have lower energy, whereas gamma rays have higher energy. The spectrum shows each light’s radiation, energy level, and wavelength.

Summarize the uses of energy, including mechanical, light, thermal, electrical, and sound energy.

1. Mechanical: To move objects and complete tasks that the human body cannot. An example would be a car on a hill.
2. Light: Uses the energy stored in light particles. An example would be solar energy.
3. Thermal: Uses energy from heat; a renewable alternative to burning fossil fuels. Examples would be fire or hydrothermal power.
4. Electrical: Moves charged particles to generate electricity. An example is a phone charger connected to the wall.
5. Sound: This is when objects vibrate; used for communication. Examples would be sonar, or a drum being played.

There are differences and similarities between diffraction and refraction. Refraction is the change in direction of waves that occurs when waves travel from one medium to another. An example would be a straw in a cup of water. Refraction deals with wavelength and speed changes. Diffraction is the bending of waves around obstacles and openings. An example would be light passing through a slit.

Before the Activity

• Practice tempering chocolate by following the directions in the Holographic Chocolate Worksheet.
• Gather all necessary supplies for making holographic chocolate.
• Before Day 1, place supplies in a central location in the room. Optional: Pre-arrange supplies into piles for each group.
• Gather recycled materials for students to use in their light-powered designs. Optional: Ask students to bring in recyclables such as cardboard boxes, or empty plastic containers (cleaned).
• Before Day 5, place supplies in a central location in the room so students can see them while brainstorming their designs.
• (Optional) Make copies of the KWL Chart.

With the Students

Day 1: Introduction & Chocolate Tempering

1. Conduct the Pre-Activity Assessment to introduce the concept of light and how light behaves: Everything we see is based on light interacting with objects in different ways. Light can bounce off of objects, diffract around or bend around objects, or refract (i.e., change direction) around objects. These different interactions are perceived and processed differently by our eyes and brains. Light behaves as a wave or as a particle, depending on how you are observing it. An example of such waves would be radio waves, and an example of such particles would be photons. Photons carry an amount of energy. Unlike sound waves, which have to travel through the air, light waves require no medium to travel through and can travel very quickly. Nothing is faster than the speed of light.
2. Give students the following two questions to discuss with their table and/or shoulder partners:

• How does light behave when it comes into contact with different types of matter? (Answer: It could bounce off of, diffract around or bend around objects, or refract.)
• Would there be color if there was no light? (Answer: No. Without light, color would not be possible.)

3. Have each group share their information in a whole-class discussion.
4. Record student answers on the KWL Chart. This can be provided by the teacher, or students can create the chart in their lab notebooks. 

   

5. Have students split into groups of four if they have not already done so. Review lab safety (e.g., use of goggles, gloves, hot plate, etc.).
6. Distribute copies of the Holographic Chocolate Worksheet. Tell students to read through the procedure for tempering chocolate before allowing them to gather their supplies.
7. Have each group gather supplies for making holographic chocolate. Tell students to notate every step they take while tempering the chocolate, so they can reflect on the process.
8. Walk around to monitor students as they temper their chocolate and pour it onto the diffraction grating. Ensure that students are continuously stirring the chocolate and that the correct temperatures are reached as they proceed. Ask students to reflect on any successes and failures they encounter; tell students that this is a very real-world application in research. 

   

9. After students finish creating their holographic chocolate, have them clean up their supplies.

Day 2: Analyzing Holographic Chocolate

1. Have students collect their holographic chocolates and provide them with a flashlight or other light source to view their chocolates under light.

   

2. Give students time to complete the questions on the Holographic Chocolate Worksheet. Allow them to research information about light interactions and diffraction gratings if needed.
3. Discuss answers as a class.

Day 3: Ask and Research

1. Review the Engineering Design Process (EDP) with the students.
2. Split students into groups of two.
3. Ask: Tell students that they are engineers who are tasked with creating, designing, and testing a light-powered product. Inform students of the criteria and constraints for this design challenge:

• They can only use recycled materials.
• They will have one flashlight.
• Their design must use light energy (either natural or via the flashlight).

Research: Give students ~30 minutes to research how light-powered products work. Encourage them to search for various types of products and record their findings.

Day 4: Imagine

1. Imagine: Have students use their research to brainstorm designs for their light-powered product. Encourage them to be creative and to record any ideas they have.

• Tell students that this is the time to think of wild ideas and defer judgement. Tell them they should discuss ideas one at a time, and that they can build on the ideas of others.
• If students seem to be struggling, tell them to return to the Research step of the EDP. Inform students that engineers often conduct research throughout the EDP. If needed, scaffold their research by directing them to the Five Easy Solar Power Experiments website.

Day 5: Plan & Create

1. Plan: Tell students to review their ideas and select a promising design for their light-powered product.

• Remind them of the criteria and constraints:
1. They can only use recycled materials.
2. They will have one flashlight.
3. Their design must use light energy (either natural or via the flashlight).
• Have students create detailed plans for their designs that include a list of the materials they will use.

2. Once students decide on a plan, allow them to collect the materials.
3. Create: Have students build prototypes of their designs. Walk around to monitor students and ask questions about why they selected the designs they did, and how the product will be powered using light.

Day 6: Create & Test

1. Let students finish creating their prototypes (if needed).
2. Test: Tell students to test their designs using the flashlight. Have them evaluate the prototype by analyzing what works, what doesn’t, and what could be improved. Remind them to record their analyses. 

   

Days 7-10: Improve

1. Improve: Have students use the results from their testing to discuss how to improve their designs. Tell students that, like engineers, they are iterating on their designs to make the best light-powered product they can.

• Tell students to record their improved designs, including how they differ from the original and whether there are any new materials they will use.
• Have students create their redesigned prototypes, re-test them, and re-evaluate the designs. Allow students plenty of time to iterate their designs. 

  

Day 11: Prepare to Communicate

1. Tell students to pretend that they are trying to pitch their product to get a solar company to purchase it or provide funding for further research to make it applicable to the real world (e.g., using non-recycled materials). Tell them that these elevator-pitch-style presentations will be 2-5 minutes long.
2. Tell students that their pitches must include 1) An explanation of how their design uses light to work, and (2 how it relates to the light-matter interactions discussed after they made holographic chocolate. Remind them that engineers communicate their designs with others to share their products and receive feedback.
3. Give students time to prepare for their presentations. Allow them to review their notes from the holographic chocolate part of the activity, and to conduct further research on the properties of light and light-matter interactions as needed.

Day 12: Communicate

1. Give students about 5 minutes to review their notes and prepare for their presentations.
2. Have students present their pitches to the class. To facilitate feedback from the class, ask the audience to share two “glows” (i.e., compliments) and one “grow” (e.g., constructive criticism) for each presentation.

Vocabulary/Definitions

absorb: When light is taken in by a medium.

diffraction: The process by which a beam of light or other system of waves is spread out as a result of passing through a narrow aperture or across an edge, typically accompanied by interference between the waveforms produced.

light: Electromagnetic radiation; a form of energy containing particle-like photons with wavelike properties.

reflect: Light bouncing off a surface, where the angle of reflection is equal to the angle of incidence.

refract: When light bends or changes direction once it comes into contact with a different medium; this is due to the change in the speed of light.

spectroscopy: The branch of science concerned with the investigation and measurement of photophysical properties due to light-matter interactions.

wavelength: The distance between successive crests of a wave; the length of a period within an oscillation.

Assessment

Pre-Activity Assessment

KWL Chart: At the beginning of Day 1, introduce the concept of light and how it behaves. As students discuss in their groups, they complete a KWL Chart. Students also brainstorm ways to demonstrate the properties of light.

Drawing Light: Have students illustrate each of the ways that light can travel. Ensure that they draw examples of reflection, diffraction, and refraction.

Activity Embedded (Formative) Assessment

Light & Chocolate: Students complete the Holographic Chocolate Worksheet to explore how light interacts with matter.

Design Process: Observe students as they create their light-powered product to ensure that they work through the whole engineering design process.

Post-Activity (Summative) Assessment

Reflection: Once students have created their light energy products, they will reflect on their designs. What about their design (and redesign(s)) worked or didn’t work? How could they improve it? Have students write a reflection about their experience with the design process.

Making Sense Assessment: Have students reflect on the science concepts they explored and/or the science and engineering skills they used by completing the Making Sense Assessment.

Investigating Questions

How can light (an abundant, renewable resource) be used as a replacement for forms of energy that require fossil fuels? (Answer: Light energy is used in many applications to produce electricity. For example, solar panels use light energy to generate electricity that can replace using fossil fuels.)

How can you demonstrate that a single ray of white light has all the colors of the spectrum? (Answer: Sunlight has the entire solar spectrum (many colors), and incandescent/fluorescent lights we use every day produce white light as well. Through the use of a diffraction grating in this activity, we can bend out the individual wavelengths so that we are able to differentiate between the various colors. We can also use prisms to illustrate this concept.)

What is special about the diffraction grating in this activity that allows for light to interact with it? (Answer: The diffraction grating has sub-micron grooves cut into the film to allow white light to be separated into multiple colors. In research, it’s separated in a specific order so that we can separate the different wavelengths for light-matter interactions and measure them. Each wavelength corresponds to multiple pixels on a camera that allows a spectrum to be measured.)

How does light interact with matter? (Answer: Light can interact with matter by exciting molecules and changing them from a grounded state to an excited state. Examples of excited states include electronic (visible light), vibrational (Infrared), rotational (microwave), etc. The light will cause a resonant interaction.)

What can you change about the surface of a material to change how it interacts with light? (Answer: Depending on the surface and texture of the material, light can reflect, diffract, or scatter. The reason that you must temper the chocolate before you apply it to the diffraction grating sheet is to create a reflective surface that can crystallize into the mold of the diffraction grating creating a holographic effect.)

Safety Issues

Holographic Chocolate:

• If students are operating the hot plates and pots, they need to be aware of how hot it can be.
• Students should wear gloves when handling chocolate.
• Students should wear goggles when tempering the chocolate.
• Students should not eat the chocolate, or lick the spatula.

Student Product:

• Students should be careful when using hot glue guns and scissors.

Troubleshooting Tips

It is recommended that the teacher test the holographic chocolate activity before rolling out the activity with their class to iron out any issues with tempering the chocolate and producing the holographic design.

Chocolate bars work better than chocolate chips for making tempered chocolate.

The chocolate should not stay in the refrigerator for longer than one night, as the chocolate does not retain the holographic qualities when left in the refrigerator for too long.

If students do not have access to hot plates, or if the chocolate is not tempering during testing, use candy melts and a microwave instead:

1. Give each group 100 g of candy melts. Place the candy in a microwave-safe bowl.
2. Melt the candy for 30-second intervals at 50% power in the microwave.
3. Stir with a rubber spatula after each interval.
4. Complete until the chocolate is melted and shiny.
5. Pour melted candy over the diffraction grating (about 2-3 mm thick).
6. Refrigerate for at least 20 minutes, or overnight.
7. Have students clean their supplies.

Activity Extensions

Have students explore spectroscopy in more detail. Encourage them to research spectroscopy and build their own DIY Spectrometer:

• DIY Spectrometer – A guide for students to create their own spectrometer.
• Breaking down how we collect spectroscopic information – Reading on how a spectrometer works.
• What is a Spectrometer? – Reading on how a spectrometer works.
• What is a Spectrometer? – A more detailed link that also examines how a spectrometer might be used in different types of spectrometry.

Activity Scaling

For 3rd-5th grade: Use candy melts instead of tempering the chocolate; see the Troubleshooting section for directions.

For 6th-8th grade: Focus more on the electromagnetic spectrum. Have students explore where light fits into this spectrum, and how scientists and engineers use waves to design products.

For 9th-12th grade: Take a deeper dive into exploring waves on the electromagnetic spectrum, comparing frequencies, and graphing wavelength comparison. There is also an optional Extension Activity using a DIY spectrometer.

Additional Multimedia Support

Intro to spectroscopy - 4:54-min long video discussing some of the aspects of the above reading and defining spectroscopy and its applications.

BrainPop: Light for a 5-minute overview of Light

Study Jams, Light Absorption, Reflection, and Refraction

“The Science and Engineering Behind the Optics” What is Light?

Optics for Kids Web Site, the first page, “What is Light?”

Discovery Learning Physical Sci: Light full Video 20 Min.

Discovery Learning Exploring Light and Color: Full Video 30 Min.

Research for light uses: http://www.darvill.clara.net/emag/emagvis.htm

Science Facts for Light: https://www.sciencekids.co.nz/sciencefacts/light.html

Scavenger Hunt different resources: http://www.3x3links.com/lightenergy

Teachers' Domain: Refraction of Light Demonstration (Requires Flash)

References

Copyright

Contributors

Supporting Program

Acknowledgements

This material is based upon work supported by the National Science Foundation under grant no. EEC-1160494—a Research Experience for Teachers program titled “Nanomanufacturing Systems for Mobile Computing and Mobile Energy Technologies, or NASCENT.” Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.