Lesson Plan

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Lesson Plan
Grade: Date: 25/02/2026
Subject: Physics
Lesson Topic: recall and use the inverse square law for radiant flux intensity F in terms of the luminosity L of the source F = L / (4πd 2)
Learning Objective/s:
  • Recall the inverse‑square law for radiant flux intensity.
  • Derive the law from isotropic emission and spherical geometry.
  • Apply the law to calculate distances or luminosities of standard candles.
  • Compare fluxes of objects with equal luminosities to infer relative distances.
  • Explain how interstellar extinction can cause the law to underestimate true distances.
Materials Needed:
  • Projector or interactive whiteboard
  • Slide deck with derivation and worked example
  • Handout worksheet containing practice questions
  • Scientific calculators or spreadsheet software
  • Graph paper for sketching diagrams
  • Laser pointer (optional) to illustrate isotropic emission
 Introduction:
Begin with a short video of a distant supernova explosion and ask how astronomers determine its distance. Review the definitions of luminosity, flux, and distance from previous energy‑transfer lessons. State that by the end of the lesson students will be able to use the inverse‑square law to find distances to standard candles and explain its underlying assumptions.
Lesson Structure:
  1. Do‑now (5') – Mini‑quiz on L, F, and d to activate prior knowledge.
  2. Derivation discussion (10') – Teacher walks through the four‑step geometric derivation on the board; students give thumbs‑up for each step.
  3. Worked example (15') – Step‑by‑step calculation of a Type Ia supernova distance; students follow in notebooks and compare results.
  4. Group activity (15') – Pairs solve two practice problems (Cepheid distance, supernova flux ratio) using calculators; teacher circulates for support.
  5. Concept check (5') – Whole‑class clicker questions on limitations such as interstellar extinction.
  6. Exit ticket (5') – Each student writes one sentence summarising when the inverse‑square law would underestimate distance.
Conclusion:
Recap that the inverse‑square law links intrinsic luminosity to observed flux, providing a powerful tool for measuring astronomical distances. Collect exit tickets to gauge understanding and address any lingering misconceptions. For homework, assign three additional problems involving different standard candles and a brief paragraph on how extinction affects the distance calculation.