Light Spreading Out and Mixing

Active learning lets students see light’s wave behavior directly through hands-on experiments. When they manipulate pinholes, slits, and colored lights, the abstract concepts of diffraction and additive color become concrete and memorable.

• 5th Year
• Principles of the Physical World: Senior Cycle Physics
• 4 activities
• 20 min–40 min

Learning Objectives

1. Explain the phenomenon of diffraction as light spreads through a narrow aperture.
2. Compare the additive mixing of primary colors of light (red, green, blue) to produce secondary colors (cyan, magenta, yellow) and white light.
3. Analyze experimental results to determine the relationship between aperture size and the degree of light spreading.
4. Demonstrate the principle of superposition by predicting and observing the resulting color when two different colored lights overlap.

Experiential Learning

⏱ 35 min·Pairs

Pairs: Pinhole Diffraction Setup

Pairs tape aluminum foil over a cardboard tube and puncture tiny holes of varying sizes. Shine a laser pointer through each hole onto a wall in a dark room. Measure and sketch the spreading pattern diameters, noting how smaller holes increase spread. Record slit width versus pattern size in a data table.

Prepare & details

What happens to light when it shines through a tiny hole?

Facilitation Tip: During Pinhole Diffraction Setup, remind students to keep the laser and screen distances consistent so they can compare results across trials.

Setup: Varies; may include outdoor space, lab, or community setting

Materials: Experience setup materials, Reflection journal with prompts, Observation worksheet, Connection-to-content framework

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

Experiential Learning

⏱ 40 min·Small Groups

Small Groups: LED Color Mixing Board

Provide red, green, and blue LED flashlights. Groups shine overlapping beams on a white poster board and predict colors like red plus blue for magenta. Photograph results and label primary, secondary combinations. Discuss why mixtures differ from paint.

Prepare & details

What happens when you shine a red light and a blue light together?

Facilitation Tip: For LED Color Mixing Board, have students test each color alone first before combining pairs to build a clear understanding of additive mixing.

Setup: Varies; may include outdoor space, lab, or community setting

Materials: Experience setup materials, Reflection journal with prompts, Observation worksheet, Connection-to-content framework

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

Experiential Learning

⏱ 25 min·Whole Class

Whole Class: Hair as Diffraction Grating

Dim lights and project a laser beam through a single classmate's hair held taut across the path. Observe twin bright spots flanking the main beam on a screen. Measure spot separation and repeat with different hair thicknesses. Class shares predictions on bending.

Prepare & details

Can light bend around corners?

Facilitation Tip: When using Hair as Diffraction Grating, encourage students to hold the hair taut to create even spacing for clearer fringe patterns.

Setup: Varies; may include outdoor space, lab, or community setting

Materials: Experience setup materials, Reflection journal with prompts, Observation worksheet, Connection-to-content framework

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

Experiential Learning

⏱ 20 min·Individual

Individual: Razor Edge Shadow Check

Each student uses a flashlight and straight razor blade to cast a shadow on paper. Examine the edge for fuzziness and sketch. Compare to geometric shadow prediction. Note diffraction evidence at the boundary.

Prepare & details

What happens to light when it shines through a tiny hole?

Facilitation Tip: In Razor Edge Shadow Check, ask students to slowly move the razor closer to the screen to observe the gradual transition from sharp to fuzzy edges.

Setup: Varies; may include outdoor space, lab, or community setting

Materials: Experience setup materials, Reflection journal with prompts, Observation worksheet, Connection-to-content framework

• Apply
• Analyze
• Evaluate
• Self-Awareness
• Self-Management
• Social Awareness

Teaching This Topic

Teach this topic by letting students explore before formal explanation. Start with observations, then introduce terms like diffraction and interference only after they’ve seen the phenomena. Avoid lectures on wave theory upfront—let the experiments drive the concepts. Research shows that students retain wave behavior better when they measure and compare results rather than just observe demonstrations.

What to Expect

Students will describe how light spreads around edges and through small openings, explain why different pinhole sizes create different patterns, and predict color outcomes when mixing lights. They should connect these observations to wave interference and wavelength relationships.

Watch Out for These Misconceptions

Common Misconception

During Pinhole Diffraction Setup, watch for students who assume larger pinholes always produce brighter patterns without considering spreading.

What to Teach Instead

Ask students to measure both the brightness and the spread of the pattern for each pinhole size, then compare the area of illumination to challenge their assumption.

Common Misconception

During LED Color Mixing Board, watch for students who think mixing all colors of light creates black like paint does.

What to Teach Instead

Have students test the combination of red, green, and blue lights first, then ask them to explain why their result differs from mixing paints before guiding them to the concept of additive color.

Common Misconception

During Hair as Diffraction Grating, watch for students who believe light passes straight through thin materials without bending.

What to Teach Instead

Have students measure the distance between fringes for different hair thicknesses and ask them to compare it to the wavelength of the laser light to show the relationship between gap size and bending.

Assessment Ideas

Exit Ticket

After Pinhole Diffraction Setup, provide students with a diagram of two pinholes of different sizes and ask them to sketch the expected patterns and explain the reason for the difference in spreading.

Quick Check

During LED Color Mixing Board, circulate and ask small groups to predict what color they expect to see when red and blue lights overlap, then observe if their prediction matches the result and discuss the wave explanation.

Discussion Prompt

After Hair as Diffraction Grating, pose the question, 'Why do shadows have fuzzy edges?' and facilitate a class discussion where students use diffraction evidence from their observations to explain the phenomenon.

Extensions & Scaffolding

• Challenge students to predict and test how green and blue light would mix during LED Color Mixing Board before they try it.
• Scaffolding for Pinhole Diffraction Setup: Provide a template with marked pinhole sizes for students who struggle with measuring diameter accurately.
• Deeper exploration: Have students research how diffraction gratings are used in spectrometers and design their own simple version using a CD and a laser.

Key Vocabulary

Diffraction	The bending and spreading of light waves as they pass through a narrow opening or around an obstacle.
Superposition	The principle stating that when two or more waves overlap, the resultant displacement at any point is the vector sum of the displacements due to each individual wave.
Additive Color Mixing	A process where different colored lights are combined, with each color adding to the overall light spectrum, leading to lighter colors.
Wavelength	The distance between successive crests of a wave, corresponding to different colors of light.