Tuesday’s Lesson

The Color of Light

By Robert Tinker

Why do we see a red rose as red and its leaves as green? What’s different about yellow and purple polka dots? What determines the colors we see? Questions like these thoroughly baffled Aristotle, Newton, and many other early scientists. Now, using a few simple models, even young learners can outsmart these great thinkers.

Newton was the first to realize that to understand fully the color of things, we need to consider separately the colors in light and the effect of roses, leaves, and other objects on the light. Use the following short activities with your students to explore the color of light. Each activity was developed in our “Do It Yourself” system and exploits a different model: Molecular Workbench, NetLogo, and Physics Education Technology. Newton would certainly have liked them!

Part 1: Light reflection

The first activity introduces a particle model of light and the idea that white light is a collection of packets of energy called photons. Photons are, of course, unusual particles because they have no mass. Light is not usually represented this way—it is usually described as a wave. A rain shower of colored particles is also a legitimate representation and much easier to understand than waves. Of course, if students ask about waves, you have the perfect opportunity to talk about wave-particle duality and the incredible idea that both representations are valid.

Go to www.concord.org/resources and run the “Light reflection” activity.

This activity features a simple NetLogo model. View the model by clicking the buttons marked “Setup” and then “Run.” This model depicts photons as red, green, and blue arrowheads (see figure 1). Encourage students to explore how reflection and absorption of photons determines the colors of objects.

1. Have students follow individual photons by slowing down the model (use the slider above the model).
2. Ask students to make predictions about the color that will be reflected as they use the red slider to absorb red photons.
3. Try the same with the green and blue sliders.

The big idea is that photons are absorbed and then some are re-emitted in random directions. The color we see is determined by these re-emitted photons. If a majority of the photons coming from a spot are red, we say that the spot looks red. If they are all absorbed, it looks black.

Part 2: Light and atoms

In the second activity, students look more closely at what happens when photons hit a solid. If the solid is colored, some photons vanish and others get re-emitted. Why?

Run the “Light and atoms” activity at www.concord.org/resources.

This activity uses a Molecular Workbench model in which atoms can be bombarded with photons (see figure 2). The big idea is that atoms sometimes absorb energy from photons and become excited. Have students explore what happens to that energy. It can result in an emitted photon or it can be converted into heat energy. Students should understand that the fates of different colored photons that interact with atoms determine the color we see.

Part 3: Neon and fluorescent lights

In this final activity, which uses one of the Physics Education Technology (PhET) models created at the University of Colorado, students look at how colored photons are made. They explore different gases in “neon” lights and watch as a single atom that is bombarded with electrons can be excited and then emit a photon if the electron has enough energy (see figure 3).

Run the “Neon and fluorescent lights” activity at www.concord.org/resources.

Ask students to make a prediction and then experiment with the model.

The big idea here is that electrical energy from a battery is converted into light by interactions between electrons and gas atoms. The detailed properties of the gas atoms determine the color of the light emitted.

It is a complex chain of events. Electrons boil off a heated electrode in one end of a tube. Inside the tube are gas atoms. If there is an electric field created by a battery, the electrons are accelerated and slam into the gas atoms, giving them energy. The amount of energy that an atom can absorb is determined by the available empty electron states.

An excited atom can lose its energy by emitting a photon of light. If that photon carries away energy in a certain range, we perceive it as colored. Thus the available excited states determine the color we see.

Ask your students to describe how they think “neon” or gas discharge lights work, and why they come in different bright colors.

1. Have students experiment with different battery voltages and atoms to create different colors.
2. Challenge students to use the configurable option for atom type and make an atom with no visible spectrum, or one whose main color is blue, green, or red.

Customizing activities

While Newton would surely have liked these activities, chances are he would have wanted to make some changes to address his own students as well as the local curriculum. You may, too. For instance, you may want to explain the idea of “wavelength,” which is avoided in our treatment.

It is easy to customize these activities or create new ones.

1. First, register at http://itsidiy.concord.org (it’s free!).
2. Load one of these activities—or any of hundreds of existing activities—in your browser.
3. Click “Copy,” which generates your own copy of the activity. You can then edit, save, and run your revised version.

This system epitomizes the decentralized “Do It Yourself” approach of our Information Technology in Science Instruction project. Student activities of the future will not be handed down from a distant expert. Instead, teams of teachers, educators, scientists, and even students themselves will develop them collaboratively.

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Robert Tinker (bob@concord.org) is President of the Concord Consortium.