Instructor demonstrating science experiment
Teacher Guide

Energy of Light

What is the difference between red light and blue light?

What is the difference between red light and blue light?

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: Energy of Light

What is the difference between red light and blue light?

  • Phosphorescent paper
  • Red LED light
  • Blue LED light
  • Button battery
  • UV LED light (Optional)

Students start by discussing the questions, “What do you think makes red light different from blue light? Does one have more energy than the other? How could we find out?” Students will then engage in an experiment where they explore red and blue lights to answer the key question, “What makes red light different from blue light? And what does this have to do with Einstein’s Nobel Prize?”

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

  • Total time
    30 - 45 minutes
  • Education level
    Grades 5 - 9
  • Content Area
    Waves, energy
  • Educational topic
    Waves, energy, light

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 it 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

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.

  • 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
Objective

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

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

  • Use the Turn and Talk protocol to ask any or all of the following set up questions.

    1. Pair students up.
    2. Give them one minute to think quietly.
    3. Give students two minutes to discuss their thinking.
    4. 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.)

  • 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.


During the experiment
  • 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?

  • 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.

  • 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!

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
  • Use the Discussion Diamond protocol.

    1. 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.
    2. The small groups should have one person from each experiment group, that way each student is coming in with different (but possibly similar) ideas.
    3. 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.
    4. 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?”
    5. All students get three minutes to think and write their thoughts in their respective corners.
    6. The students take turns explaining their ideas to each other (all students must share).
    7. The students discuss what their consensus view might be and write their consensus view in the middle.
  • 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.

  • Have students learn 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.

    1. Einstein and the Photoelectric Effect
    2. Wave-Particle Duality and the Photoelectric Effect
    3. Quantum Mechanics - Part 1: Crash Course Physics #43
    4. Light Is Waves: Crash Course Physics #39
  • 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.

Teacher tip

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.

  • Real world connections:
    • Humans can only see the visible part of the electromagnetic spectrum, but certain animals can see light at different wavelengths. Use this Ask a Biologist website to discover the different ways different animals see light, and present your research in a short podcast.
    • Another popular application of the photoelectric effect is solar cells. Using this helpful NASA website and based on what you learned in this activity, create a short presentation or podcast about how solar cells work.
  • Suggestions for drawing, illustrating, and presenting content in creative ways.
  • Give each student a pair of diffraction glasses and have them look at various types of light with them on. Students can draw what they are seeing, and then have them explain how the glasses are separating the light.
    • Students should learn that these glasses separate light by its different wavelengths/energies.
  • Engineering and design challenges connected to the content:
    • A lot of different STEM fields use properties of light to categorize different objects based on wavelength/energy. Think of a way you can use different properties of light to discover something new.
      • 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!

Physicists To-Go

Sign up for Physicists To-Go to have a scientist talk to your students

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-PS4-1
    Use mathematical representations to describe a simple model for waves that includes how the amplitude of a wave is related to the energy in a wave.
  • MS-PS4-2
    Develop and use a model to describe how waves are reflected, absorbed, or transmitted through various materials.
  • MS-PS4.A
    Wave Properties

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

PhysicsQuest © 2023 by American Physical Society is licensed under CC BY-NC 4.0

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