Lesson Plan

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Lesson Plan
Grade: Date: 17/01/2026
Subject: Physics
Lesson Topic: Define and use the terms focal length, principal axis and principal focus (focal point)
Learning Objective/s:
  • Define the principal axis, principal focus and focal length for thin lenses.
  • Apply IGCSE sign conventions to determine object, image and focal distances.
  • Construct accurate ray diagrams using the three standard rays.
  • Use the lens formula to calculate image distance and describe image characteristics.
Materials Needed:
  • Projector and screen
  • Convex and concave lens set
  • Ray‑diagram worksheets
  • Rulers and calculators
  • Printed handout of definitions and sign conventions
 Introduction:
Begin with a quick demonstration of a flashlight passing through a convex lens to form a bright spot, prompting students to consider why the light converges. Review prior knowledge of refraction and straight‑line propagation. Explain that today they will master the terminology of focal length, principal axis and principal focus and will apply these in calculations and diagrams.
Lesson Structure:
  1. Do‑now (5'): Students label parts of a provided lens diagram (principal axis, focus, centre).
  2. Mini‑lecture (10'): Define terms, sign conventions, and introduce the lens formula.
  3. Guided practice (12'): Work through the example problem, completing steps on a worksheet.
  4. Ray‑diagram activity (10'): In pairs, draw the three standard rays for a given object distance; teacher projects a model for checking.
  5. Quick quiz (8'): Three short questions on terminology and sign usage (exit ticket).
  6. Recap & homework (5'): Summarise key points and assign a worksheet on lens calculations.
Conclusion:
Recap how the principal axis, focus and focal length interrelate and guide ray‑diagram construction. Collect exit tickets to gauge understanding. For homework, complete the worksheet calculating image distances for a mix of convex and concave lenses.