Lesson Plan

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Lesson Plan
Grade: Year 12 Date: 17/01/2026
Subject: Physics
Lesson Topic: recall and use λ = ax / D for double-slit interference using light
Learning Objective/s:
  • Describe the double‑slit interference setup and identify key variables (a, x, D, λ, m).
  • Apply the small‑angle approximation to derive λ = a x / D for the first‑order bright fringe.
  • Use the formula to calculate wavelength, slit separation, fringe spacing or screen distance, showing correct unit conversion and significant figures.
  • Check the validity of the approximation and evaluate results for higher‑order fringes.
Materials Needed:
  • Projector or interactive whiteboard for diagrams and derivations.
  • Double‑slit demonstration kit or virtual simulation.
  • Worksheet with practice questions and data tables.
  • Calculator or spreadsheet for unit conversion and calculations.
  • Ruler or measuring tape for measuring fringe spacing (if physical demo).
 Introduction:
Begin with a quick demonstration of a double‑slit pattern projected onto the screen to capture interest. Ask students what information can be extracted from the fringe spacing and how it relates to the light’s wavelength. Remind them of the path‑difference condition and state that by the end of the lesson they will be able to recall and correctly apply λ = a x / D.
Lesson Structure:
  1. Do‑now (5'): Students sketch a double‑slit pattern and list the variables they think are relevant.
  2. Mini‑lecture (10'): Derive λ = a x / D using the small‑angle approximation, emphasising assumptions.
  3. Guided practice (12'): Work through the example problem together, converting units and calculating λ.
  4. Independent practice (15'): Students solve two additional A‑level style questions on the worksheet while the teacher circulates.
  5. Check for understanding (5'): Quick exit‑ticket question – write the formula for λ and state when it is valid.
  6. Summary discussion (3'): Review common pitfalls and answer any lingering questions.
Conclusion:
Summarise that the λ = a x / D relation links measurable geometry to the light’s wavelength when the small‑angle condition holds. Collect the exit tickets to gauge understanding, and assign homework to complete a set of extra problems requiring unit conversion and significance‑figure justification. Remind students to verify the approximation before using the formula.