Diffraction Patterns

Overview

In this lesson (13 of 25) students use the PhET “Wave Interference” simulation to explore diffraction, then connect simulation observations to real-world diffraction patterns using a diffraction grating. Students design and compare both virtual and real investigations to explain how wave effects change pattern shape.

Learning intentions

• Students will investigate diffraction by observing how changing wavelength and grating spacing affects the pattern.
• Students will distinguish diffraction effects from simple interference features using evidence from data.
• Students will design a fair experiment (virtual and real) and justify what variables are changed, controlled, and measured.
• Students will use scientific process skills to interpret results and communicate explanations.

Success criteria

• I can describe diffraction patterns and relate them to wave behaviour.
• I can identify independent, dependent, and controlled variables in my investigation.
• I can compare virtual and physical results and explain similarities/differences using wave ideas.
• I can present my method and findings clearly (tables/graphs/annotated diagrams).

Curriculum links

• PY-11-02 — Students explain the properties and behaviours of waves (including wave behaviours relevant to diffraction and interference).
• PY-11WS-06 — Students use scientific process to solve scientific problems by planning, conducting, and communicating investigations.
• PY-11WS-04 — Students select and process qualitative and quantitative data by recording pattern features and measurements.
• PY-11WS-02 — Students design and evaluate scientific investigations by making and testing decisions about variables and method.

Lesson structure (45 minutes)

1. 0–5 min · Hook (pattern observation). Teacher displays/reads a short prompt: “When a wave meets an obstacle or passes through a grating, why don’t we get just a sharp line?” Students quick-write a prediction for what changes in a diffraction pattern when spacing changes.

2. 5–12 min · Mini-teach (diffraction essentials). Teacher explains that diffraction is a wave behaviour where spreading occurs when waves interact with edges or gratings; pattern size/spacing depends on wavelength relative to grating spacing. Students annotate a simple diagram: grating → central maximum → side maxima/minima, noting “spread increases” in words (no maths required).

3. 12–22 min · Virtual investigation setup (PhET). Teacher models one controlled test in PhET “Wave Interference”: selecting conditions and adjusting parameters systematically while recording observations (pattern width/angle positions of maxima). Students in pairs complete Investigation Sheet A: run 3 trials with one variable changed (choose either wavelength or grating spacing), holding others constant, and record pattern features in a table.

4. 22–30 min · Prediction challenge (technology-to-physical connection). Teacher asks: “Based on your virtual trend, predict what the physical pattern should do if we change the grating or laser colour.” Students answer individually using a sentence starter: “If wavelength/grating spacing increases, then diffraction ____ because…”. Teacher circulates for misconceptions (e.g., students confusing intensity with position).

5. 30–40 min · Real diffraction test (laser + grating). Teacher distributes diffraction gratings and sets up a bench station: laser pointer with a grating mounted; a screen (paper) at a measured distance. Students in small groups carry out one physical trial based on their earlier virtual plan, using a table to record qualitative pattern features (number of visible maxima, relative spacing, width of central maximum) and one measurement if feasible (e.g., distance between two bright maxima on the screen). Teacher ensures safe laser practices and supervision.

6. 40–45 min · Share & exit ticket (evidence-based explanation). Students complete a 3-part exit ticket:

• claim: “My pattern changed in this way…”
• evidence: “From virtual/real data, I observed…”
• reasoning: “This supports diffraction because…” Teacher selects two responses to discuss briefly, linking back to wave behaviours.

Resources

• PhET “Wave Interference” simulation access on student devices (no web links needed in class instructions)
• Investigation Sheet A (virtual variables table + observation prompts)
• Investigation Sheet B (physical method, results table, quick diagram space)
• Diffraction grating(s) (clear identification of grating spacing type)
• Laser pointer(s), grating holders/mounts, screen materials (paper/cardboard)
• Rulers/tape measures, markers for labelling screen positions
• Safety instructions posted and reviewed (laser safety signage and procedures)

Assessment

• Formative: teacher checks virtual data tables for controlled-variable thinking and accurate recording of pattern features.
• Formative: observe group discussions during prediction challenge for correct wave reasoning (wavelength vs grating spacing).
• Summative for today (exit ticket): assess whether students make evidence-based claims connecting diffraction to wave behaviours and use scientific process language.

Differentiation

• Support for students needing structure: provide sentence starters for variable descriptions and explanation (“I changed… I kept… I measured…”).
• Support for data handling: offer a template with example rows for recording maxima positions or pattern width categories.
• Extension for fast finishers: ask them to propose an additional virtual trial that would distinguish diffraction from interference (e.g., altering which setup feature is responsible for pattern formation) and to predict the outcome.
• EAL/SEN considerations: allow diagrams with labels as acceptable “evidence”, and provide a word bank (diffraction, wavelength, spacing, maxima, minima, central maximum, spread).