Lesson Plan

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Lesson Plan
Grade: Date: 17/01/2026
Subject: Physics
Lesson Topic: State the meaning of critical angle
Learning Objective/s:
  • Define the critical angle and explain its significance in refraction.
  • Derive and use the formula sin c = n₂/n₁ to calculate critical angles for given media.
  • Predict when total internal reflection will occur based on the angle of incidence.
  • Compare critical angles for common material pairs and relate them to differences in refractive index.
Materials Needed:
  • Projector or interactive whiteboard
  • Physics textbook or handout on refraction
  • Ray‑tracing worksheets
  • Printed refractive‑index data table
  • Laser pointer or ray‑model simulation software
  • Whiteboard markers
 Introduction:
Begin with a quick demonstration: shine a laser from water into air and observe the beam skimming the surface. Ask students what determines the angle at which this occurs, linking to prior knowledge of Snell’s law. Explain that today they will articulate the definition of the critical angle and learn how to calculate it. Success will be measured by correctly deriving the formula and solving sample problems.
Lesson Structure:
  1. Do‑now (5’) – Students answer a short question on Snell’s law on the board.
  2. Mini‑lecture (10’) – Define critical angle, display diagram, derive sin c = n₂/n₁.
  3. Guided practice (12’) – Calculate critical angles for water‑air, glass‑air, and diamond‑air.
  4. Interactive simulation (8’) – Use a ray‑tracing app to vary incidence angles and observe total internal reflection.
  5. Concept check (5’) – Quick quiz (exit ticket) with two problems on identifying TIR conditions.
  6. Summary discussion (5’) – Review key points and address misconceptions.
Conclusion:
Recap that the critical angle marks the transition to total internal reflection and can be found with sin c = n₂/n₁. Have students write an exit‑ticket answer describing a real‑world example of total internal reflection. Assign homework: calculate the critical angle for three additional material pairs and explain the observed trend. End with a reminder to review the refractive‑index table for the next lesson.