Lesson Plan

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Lesson Plan
Grade: 9 Date: 17/01/2026
Subject: Physics
Lesson Topic: Describe the action of thin converging and thin diverging lenses on a parallel beam of light
Learning Objective/s:
  • Describe how a thin converging lens refracts a parallel beam to a real focal point.
  • Explain how a thin diverging lens refracts a parallel beam to a virtual focal point.
  • Apply the sign convention for focal lengths when analysing lens behaviour.
  • Construct accurate ray diagrams for both lens types to predict image location.
  • Compare the image characteristics produced by converging and diverging lenses.
Materials Needed:
  • Thin convex and concave lens kits
  • Ray box or laser pointer to produce a parallel beam
  • White screen or sheet of paper
  • Ruler and protractor
  • Projector with slide of ray diagrams
  • Worksheet with diagram and comparison tasks
 Introduction:
Begin with a quick demonstration: shine a laser through a convex lens onto a screen to show a bright focal point. Ask students what they notice about the beam before and after the lens, linking to prior work on refraction. Explain that today they will explore how both converging and diverging lenses affect parallel light and how to predict the resulting image.
Lesson Structure:
  1. Do‑now (5'): Mini‑quiz on focal‑length sign conventions.
  2. Teacher demonstration (10'): Convex lens focusing parallel rays; discuss real focal point.
  3. Guided practice (12'): Pairs use ray boxes and lenses to draw ray diagrams for a converging lens, checking against the model.
  4. Diverging lens demo (8'): Repeat with a concave lens, emphasizing the virtual focal point.
  5. Comparative activity (10'): Groups complete a comparison table (lens type, shape, sign of f, beam effect, image).
  6. Check for understanding (5'): Exit question – “Where does the image form for a parallel beam through a diverging lens?”
  7. Summary & homework (5'): Recap key points; assign worksheet problems for further practice.
Conclusion:
Review the main differences between converging and diverging lenses regarding focal‑point location and image type. Have students write one sentence on an exit ticket describing which lens produces a real focal point and why. Remind them to complete the worksheet problems for homework to reinforce ray‑diagram skills.