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.

• Grade 12
• Physics
• 4 activities
• 25 min–40 min

Learning Objectives

1. Explain the relationship between the refractive index of a medium and the wavelength of light.
2. Analyze the path of light rays through a prism to predict the order of colors in a spectrum.
3. Calculate the angle of refraction for light entering and exiting a raindrop using Snell's Law, considering dispersion.
4. Compare the angular separation of colors in a primary rainbow versus a secondary rainbow.
5. Synthesize information about dispersion and total internal reflection to construct a ray diagram for a rainbow.

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.

Prepare & details

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.

Setup: Groups at tables with access to source materials

Materials: Source material collection, Inquiry cycle worksheet, Question generation protocol, Findings presentation template

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

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.

Prepare & details

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.

Setup: Groups at tables with access to source materials

Materials: Source material collection, Inquiry cycle worksheet, Question generation protocol, Findings presentation template

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

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.

Prepare & details

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.

Setup: Groups at tables with access to source materials

Materials: Source material collection, Inquiry cycle worksheet, Question generation protocol, Findings presentation template

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

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.

Prepare & details

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.

Setup: Groups at tables with access to source materials

Materials: Source material collection, Inquiry cycle worksheet, Question generation protocol, Findings presentation template

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

Teaching This Topic

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.

What to Expect

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

Common Misconception

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

What to Teach Instead

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.

Common Misconception

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

What to Teach Instead

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.

Common Misconception

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

What to Teach Instead

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.

Assessment Ideas

Quick Check

After the Prism Lab, present students with a diagram of white light entering a prism and ask them to draw the path after passing through, labeling at least three colors and indicating which bends most and least.

Discussion Prompt

During the Outdoor Hose Rainbow activity, pose the question: 'Why is the sky often clear blue above the 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.

Exit Ticket

After the Water Tank Rainbow activity, ask students to write a short paragraph explaining the difference between primary and secondary rainbows, focusing on the number of internal reflections and the resulting color order.

Extensions & Scaffolding

• Challenge rapid finishers to predict the angle at which a secondary rainbow would appear using ray diagrams and prisms.
• Scaffolding for struggling students: provide pre-labeled prism diagrams with missing color orders to complete and color-code.
• Deeper exploration: ask students to calculate the approximate angle of deviation for red and violet light using the prism data they collect.

Key Vocabulary

Dispersion	The phenomenon where white light separates into its constituent colors when passing through a medium, due to the medium's refractive index varying with wavelength.
Refractive Index	A measure of how much light bends, or refracts, when entering a medium. It is dependent on the wavelength of the light.
Total Internal Reflection	The complete reflection of light at the boundary between two media, occurring when the angle of incidence exceeds the critical angle and light travels from a denser to a less dense medium.
Spectrum	The range of colors produced when white light is dispersed, ordered by wavelength from violet (shortest) to red (longest).