Physics · Grade 12

Active learning ideas

Dispersion and Rainbows

Active learning lets students manipulate light directly, turning abstract wave optics into visible, repeatable phenomena. By handling prisms and water tanks, they transform equations into observed arcs and remembered patterns, making dispersion concrete rather than abstract.

• Ontario Curriculum Expectations
• HS.PS4.B.1

• 25–40 min
• Pairs → Whole Class
• 4 activities

Activity 01

• Inquiry Circle
• 35 min · Small Groups

Prism Lab: Spectrum Observation

Supply small groups with prisms, flashlights, and screens. Direct students to shine white light through the prism at varying angles to produce clear spectra. Have them identify colors, sketch ray paths, and note bending differences for red and violet.

Explain how dispersion causes white light to separate into its constituent colors.

Facilitation Tip

During the Prism Lab, have students sketch ray paths before touching prisms to build predictive models, then refine drawings after observations.

What to look for

Present students with a diagram of white light entering a prism. Ask them to draw the path of the light after passing through the prism, labeling at least three colors and indicating which color bends the most and least.

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Activity 02

• Inquiry Circle
• 40 min · Pairs

Water Tank Rainbow: Droplet Simulation

Fill clear tanks with water and a drop of milk for light scattering. Pairs shine laser pointers at shallow angles to mimic raindrop entry, observing refraction, internal reflection, and exit dispersion. Record color separation and angles on diagrams.

Analyze the role of dispersion and total internal reflection in the formation of rainbows.

Facilitation Tip

In the Water Tank Rainbow, ask students to adjust the angle of incident light until colors appear clearly, linking geometry to dispersion.

What to look for

Pose the question: 'Why is the sky often clear blue above a rainbow, but red or orange below it?' Guide students to discuss the angles of refraction and reflection for different colors and the path light takes to reach their eyes from different parts of the sky.

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Activity 03

• Inquiry Circle
• 25 min · Pairs

CD Spectrometer: Diffraction Check

Individuals or pairs use CDs as gratings with white light and slits. Project spectra on walls and compare color orders to prisms. Discuss how grating dispersion mirrors prism effects, measuring approximate wavelengths if rulers available.

Predict the order of colors in a spectrum produced by a prism.

Facilitation Tip

When using the CD Spectrometer, remind students to hold the CD near a window or lamp at a consistent height to standardize diffraction patterns.

What to look for

Students write a short paragraph explaining the difference between the formation of a primary and a secondary rainbow, focusing on the number of internal reflections and the resulting order of colors.

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

Activity 04

• Inquiry Circle
• 30 min · Whole Class

Outdoor Hose Rainbow: Field Test

On sunny days, whole class positions with backs to sun and sprays mist from hoses. Groups observe primary and secondary bows, measure observer angles, and photograph color sequences for classroom analysis.

Explain how dispersion causes white light to separate into its constituent colors.

Facilitation Tip

For the Outdoor Hose Rainbow, let students spray water at different angles, noting where the rainbow appears and disappears to connect geometry to observation.

What to look for

Present students with a diagram of white light entering a prism. Ask them to draw the path of the light after passing through the prism, labeling at least three colors and indicating which color bends the most and least.

• Analyze
• Evaluate
• Create
• Self-Management
• Self-Awareness

A few notes on teaching this unit

Start with the prism lab to establish dispersion as refraction-based, then move to water tanks for spherical droplet modeling. Avoid starting with rainbows in clouds, as this reinforces the mirror misconception. Research shows that tracing rays with hands-on tools builds stronger mental models than abstract diagrams alone.

Successful learning appears when students can trace light paths through diagrams and physical models, predict color order from wavelength, and explain why red and violet refract differently. They should also distinguish primary from secondary rainbows by counting internal reflections.

---

Watch Out for These Misconceptions

• During the Outdoor Hose Rainbow activity, watch for students attributing the rainbow to cloud reflection rather than individual water droplets.

  Have students trace light from the hose to their eyes using the water droplets, then sketch the path on whiteboards to confirm no cloud involvement.

• During the Prism Lab activity, watch for students labeling red as bending more than violet.

  Ask students to measure and annotate the angle of refraction for each color using protractors and colored light projections, reinforcing the inverse relationship between wavelength and refraction.

• During the Water Tank Rainbow activity, watch for students assuming all rainbows show the same color order without considering secondary arcs.

  Have students simulate two reflections by adjusting the light source angle and sketch both primary and secondary arcs, then compare diagrams to identify reversed color orders.

---

Methods used in this brief

• Inquiry Circle