Energy of Light

Teacher’s Guide

Teacher Background

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

Albert Einstein received a Nobel prize for his explanation of the photoelectric effect. Einstein’s contribution was the realization that light had a particle nature as well as a wave nature. Light is made of individual particles or packets of energy called photons. Different colors of light have photons of different energies. Photons of red light have lower energy; photons of blue light have higher energy. Suppose you need to get a gallon of milk. You could get two half-gallon bottles, or you could get four-quart bottles. The total amount of milk is the same — but it comes in different-sized “chunks.” If you have blue light, it is like getting your light in half-gallon bottles; with red light, it is like getting your light in quart bottles.

When you charge up the phosphorescent paper included in this activity, the individual molecules within the paper can only absorb one photon at a time. If the absorbed photon has enough energy, the molecule will give back some light energy (along with some thermal energy). The amount of light energy given back is less than the amount of light energy absorbed, and so is a different color than the input light. The red photons don’t have enough energy to make the paper glow, but the photons of blue light do. At the far end of the spectrum, ultraviolet photons have a lot of energy — they are quite zesty. That’s why they can give you a sunburn! The same goes for the entire electromagnetic (EM) spectrum, even outside the range our eyes can see. The EM spectrum is a continuous gradient from low-energy radio waves (these are a type of light too!) to higher and higher energies of light, the highest of which we call gamma rays.

Key Terms (used or presented after the activities)

• Photon: A particle representing a quantum of light/
• Energy: The energy associated with a single photon of light is given by E = h. Where h is Planck’s constant and is the frequency of the radiation.
• Photoelectric effect: The emission of electrons when light hits a material.

Objectives

Students will experiment with the idea that different colors of light are not the same, but vary in wavelength and energy.*

*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!

1. Use the Turn & Talk protocol to ask any or all of the following set up questions:
   • Pair students up.
   • Give them one minute to think quietly.
   • Give students two minutes to discuss their thinking.
   • Have students record their answers or share out to the whole group.
     Do you think light has energy? How do you know?
     If light does have energy, does all light have the same amount of energy? How do you know? (Think back to other experiments we have done to use as evidence for your answers.)
2. Tell students that the experiment they are about to do will help them explore these properties of light, and discover what makes one color of light different from another.

Setting Up

You’ll need one of each color of light (red and blue) per sheet of phosphorescent paper. One set of materials per student is ideal, but small group work is certainly possible.

Tell students the day before the activity to bring any glow-in-the-dark clothes/objects/materials from home to use in our experiment!

During the Experiment

Collecting Data

1. Dim the lights in your classroom and make it as dark as you can. Notice the eerie green glow coming from the phosphorescent (glow-in-the-dark) paper. Why do you think it is glowing? Where did the energy come from to produce the glow?
2. Now, get out the red and blue lights and experiment with drawing and writing messages on the paper. Observe how the brightness of your messages changes with time, and discuss your observations of the effects of the different lights with a partner.
3. Another interesting thing: If you shine the red light on a bright spot on the paper, you may find that the spot actually gets dimmer. If the paper is warm, it glows more brightly. This means the energy is used up faster. Shining the red light on the paper can warm it up right where the red light hits; this will make the surface discharge faster, leaving a dark spot!

Important Note: Listen in on each group’s discussion, and answer as few questions as possible. Even if a group is off a little, they will have a chance to work out these stuck points later during the discussion.
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Teacher Tips

• 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 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 students from underrepresented backgrounds 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 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. Look for *** below to find suggested places to implement these techniques.

Conclusion

1. Use the Discussion Diamond protocol
   • Setup: Each small group or table needs a copy of a graphic organizer (included at the end of this guide). Each student will write one of the triangular “corners” of the organizer.
   • The small groups should have one person from each experiment group, that way each student is coming in with different (but possibly similar) ideas.
   • Show them the entire electromagnetic spectrum and have them predict if the wavelengths of these are more, less, or equal to blue and red light. Have them describe their thinking.
   • Pose the questions, “Based on what you learned in this activity, what can you infer about the energy of other colors of light? What would happen if you shine a yellow light on the phosphorescent paper? Purple light? Orange light? Radio waves? X-rays?”
   • All students get three minutes to think and write their thoughts in their respective corners.
   • The students take turns explaining their ideas to each other (all students must share).
   • The students discuss what their consensus view might be and write their consensus view in the middle.
2. Have groups share their consensus views, and emphasize how through experimentation they answered the key question and were able to infer about other types of light based on their observations. Review any key terminology as a whole group.
3. Have students read about the photoelectric effect. It’s an interesting example of when light acts more as a particle than a wave. This is what Einstein discovered — and he later won a Nobel Prize in Physics for it!
   Links:
   https://www.youtube.com/watch?v=MFPKwu5vugg&t=41shttps://www.aps.org/publications/apsnews/200501/history.cfm
4. Introduce the PQ 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.

Student’s Guide

Introduction

The different colors of light are not the same. We are going to explore and find out whether red light or blue light has more energy and what this has to do with Albert Einstein’s Nobel Prize.

Objective: Students will experiment with the idea that different colors of light are not the same but vary in wavelength and energy.

Students: After reading the introduction, what is your essential question or objective for this activity?
 

Before the Experiment

1. Do you think light has energy? How do you know?
2. If light does have energy, does all light have the same amount of energy? How do you know? (Think back to other experiments we have done to use as evidence for your answers.)

Setting Up

1. (Day before) Bring any glow-in-the-dark clothes/objects/materials from home to use in our experiment!
2. Get the phosphorescent paper, LED lights, and a button battery from your teacher.

During the Experiment***

Collecting Data

1. Dim the lights in your classroom and make it as dark as you can. Notice the eerie green glow coming from the phosphorescent (glow-in-the-dark) paper. Why do you think it is glowing? Where did the energy come from to produce the glow?
2. Now get out the red and blue lights and experiment with drawing and writing messages on the paper. Observe how the brightness of your messages changes with time, and discuss your observations of the effects of the different lights with a partner.
3. Another interesting thing: If you shine the red light on a bright spot on the paper, you may find that the spot actually gets dimmer. If the paper is warm, it glows more brightly. This means the energy is used up faster. Shining the red light on the paper can warm it up right where the red light hits; this will make the surface discharge faster, leaving a dark spot!

Analyzing Data

1. What can you say about the energy of red light compared to blue light?
2. Using colored pencils/markers, draw what you think the transverse waves for these two colors of light look like.
3. Bonus: Draw a physics model of what you think is happening INSIDE the phosphorescent paper when it is hit with red light. Blue light.

Conclusion***

1. In your discussion diamond groups, answer the following questions:
   What can you infer about the energy of other colors of light? What would happen if you shine a yellow light on the phosphorescent paper? Purple light? Orange light? Radio waves? X-rays? (It might be helpful to look at a picture of the electromagnetic spectrum while answering these questions!)
2. Read up on the photoelectric effect. It’s an interesting example of when light acts more as a particle than a wave. This is what Einstein discovered — and he later won a Nobel Prize in Physics for it!
   Different links based on the level of your students:
   https://www.youtube.com/watch?v=7kb1VT0J3DE
   https://www.youtube.com/watch?v=IRBfpBPELmE
   https://www.youtube.com/watch?v=MFPKwu5vugg&t=41s
   https://www.aps.org/publications/apsnews/200501/history.cfm
3. Explore the PQ 23: Making Waves Physics Career and Concept Map. What careers that use this content sound most appealing to you? Why?