Teacher Guide

Color Blockers

Experimenting with color absorption and transmission

What colors of light are absorbed and transmitted by different colors of water?

This resource was originally published in PhysicsQuest 2014: Quantum.

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: Color Blockers

What colors of light are absorbed and transmitted by different colors of water?

• Electromagnetic spectrum inhibitor tablets
• Diffraction grating viewer
• Red, green, and blue LEDs
• Batteries
• Flashlight
• Water
• Three clear plastic cups
• Tape

Students start by discussing what they know about color. Then, they experiment looking at colored water through different lights to help them understand and describe color absorption and transmission. Students will have a chance to explain the phenomenon of why the ocean looks blue and work together to refine their thinking.

• Total time
45 - 60 minutes
• Education level
• Content Area
Quantum
• Educational topic
Color, light

What does Quantum Mechanics mean? The first question you may have is how are rainbows related to the quantum mechanics. And what is quantum mechanics exactly?

Quantum mechanics is a way of looking at the very small things that make up atoms, like neutrons and electrons, and other small bits like photons. Things that small don’t behave the way you would think they should. They pretty much behave exactly the opposite of how you think they should but physicists can describe how they work using quantum mechanics.

What does the term “Quantum Mechanics'' actually mean? “Quantum” means something that comes in specific amounts. Electrons have a charge of 1.6x10-19 coulombs. You can only get charged in that amount, not 2.3x10-19 coulombs or 1.0x10-19 coulombs. Charge has to come in multiples of 1.6x10-19. It is “quantized.” Quantum mechanics is a way of looking at the mechanics or interactions of these small, quantized things. Charge isn’t the only thing quantized; energy is too.

In the comic book, when Dr. Hene’s minivan can only go two specific speeds but nothing in between, her energy is “quantized,” meaning it can be one way or another but nothing in between.

Light is made up of small particles called photons. When you look at a rainbow, the different colors are made up of photons with different energies. Violet has more energy than red. In fact, the energy in each photon increases as you move from red to violet, so green has more energy than yellow. The reason the rainbow splits the way it does is because of the amount of energy in each color’s photons.

Atoms are made of electrons in clouds around a nucleus. The energy of these electrons is also “quantized.” Electrons around an atom can have one energy or another but nothing in between. The way an atom is usually drawn, with electrons at specific points around a nucleus, is a good way to think of energy levels. The different orbits of the electrons represent different energy levels. There are many different energy levels and electrons can move between one and the other.

To get an electron to move from one energy to another, it has to have something add exactly the right amount of energy to kick it up to another energy level. See the image.

Just like Dr. Hene’s foot on the gas pedal caused her minivan to move from one speed right to another. For atoms this “foot on the gas” is a photon. If a photon with just the right amount of energy hits an atom, the electrons jump around. When this happens, the photon is “absorbed.” And different atoms have electrons that can have different energy levels. So the electrons in hydrogen can have different energy levels than the electrons in oxygen. Which means oxygen “absorbs” different color photons than hydrogen. Photons that don’t have the right energy to knock around the electrons will just pass right on by. They are “transmitted.”

In this activity the students will look at what happens to light as it passes through different colors of water. They will see that for colored water, only a few colors of light will pass through. The rest of the colors are absorbed by the colored water. In the case of the blue water, the molecules that make up the blue dye have the right energy levels to absorb almost all colors. Every color photon except blue and green will be absorbed. This is why the water appears blue.

Key terms

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

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

• Absorb: To take something in. In the case of light, it means an atom takes in the photon’s energy and doesn’t let the photon pass through.
• Transmit: Allow to pass through. The opposite is absorbed. Light is either absorbed or it is transmitted.
• Spectrum: The different colors of light. The “spectrum” of a blue LED is different from the “spectrum” of a red LED.
Objective

Students will experiment to understand color absorption and transmission, and they will discuss the phenomenon of a “blue ocean.”

Before the experiment
• Connect to the introduction
1. Let students experiment with food coloring in cups of water to create new colors. Ask them what it means or what is happening when new colors get created?
2. Look up the paint color combining trend and let students guess what color they think will result from the mixture.

What do you think it means when we say “different colors have different amounts of energy”? Which colors do you think have more energy/less energy?

• Snowball protocol
1. Pair students up.
2. Give them a minute to think quietly.
3. Give students 2 minutes to discuss their thinking.
4. Have students record their answers or share out to the whole group.

• Tell students that the experiment they are about to do will help them explore light absorption and transmission.

Setting up
1. Fill the three plastic cups halfway full of water.
• Color the water red, yellow and blue using the “electromagnetic spectrum inhibitor tablets.”

• Tape the flashlight to the edge of a table or chair so that it is pointing straight up.

• Place the battery between the red LED wires, making sure to put the longer leg on the positive side of the battery (the one with the “+” sign).

• Tape the lit LED to the edge of the table or chair about a foot away from the flashlight.

During the experiment
• Make sure students are put into intentional groups. See above.

• Students will complete the experiment using the Student Guide where we have outlined the experiment for students and along the way, they record results and answer questions.

Analyzing data
• In the Student Guide, they will look at their data to describe which filters allowed each color through.

• Continue to listen in on each group’s discussion; 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.

Teacher tip
1. Suggested STEP UP Everyday Actions to incorporate into activity.
1. When pairing students, try to have male/female partners and invite female students to share their ideas first.
2. As you put students into groups, consider having female or minority students take the leadership role.
3. 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.
2. Consider using white boards so students have time to work through their ideas and brainstorms before saying them out loud.
3. 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
• Follow the Write-Pass protocol to have students share and refine their thinking:

1. Divide students into groups of four (different from their experimental groups).
2. The teacher posts a question that students must answer with an explanation. Why do oceans look blue?
3. Students each write their own ideas on a loose piece of paper.
4. Then pass the papers to the left.
5. Each student silently reads the 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 until each student gets to read and comment on each other's 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.

Real world connections:

• Using what you learned in this activity about absorption and transmission, why do leaves on a tree often appear green? See the explanation on properties of light.
• Suggestions for drawing, illustrating, presenting content in creative ways:
• Select white light and one other color of water and draw a diagram of the colors of light going in versus the colors of the object being transmitted.
• Example:
• MS-PS4-2
Develop and use a model to describe how waves are reflected, absorbed, or transmitted through various materials.
• MS-PS4-3-applications
CCC: Influence of Science, Engineering, and Technology on Society and the Natural World. Technologies extend the measurement, exploration, modeling, and computational capacity of scientific investigations. (MS-PS4-3)
• MS-PS4-3-nature-of-science
CCC: Science is a Human Endeavor. Advances in technology influence the progress of science and science has influenced advances in technology. (MS-PS4-3)
• MS-PS4-1-empirical-evidence
SEPs: Scientific Knowledge is Based on Empirical Evidence. Science knowledge is based upon logical and conceptual connections between evidence and explanations. (MS-PS4-1)

Credits

Written by Rebecca Thompson, PhD

Illustrations by Kerry G. Johnson

Activity illustrations and graphics by Nancy Bennett-Karasik

Updated in 2023 by Sierra Crandell, MEd, partially funded by Eucalyptus Foundation

Extension by Jenna Tempkin with Society of Physics Students (SPS)