Lesson Plan

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Lesson Plan
Grade: Date: 17/01/2026
Subject: Physics
Lesson Topic: Describe the dispersion of light as illustrated by the refraction of white light by a glass prism
Learning Objective/s:
  • Describe how white light is separated into a spectrum by a glass prism.
  • Explain the relationship between wavelength, refractive index and refraction angle (dispersion).
  • Apply Snell’s law to predict the deviation of different colours in a prism.
  • Identify the observable features of dispersion and relate them to chromatic aberration in lenses.
Materials Needed:
  • Triangular glass prism
  • White light source (flashlight or lamp)
  • Projection screen or white paper
  • Protractor or angle‑measuring tool
  • Worksheet with spectrum table
  • Computer/projector for diagram display
 Introduction:
Begin with a quick demonstration of a flashlight shining through a prism to produce a rainbow on the screen, capturing students' curiosity. Review that white light contains many colours and that refraction changes a ray’s direction when it enters a new medium. Explain that today’s success criteria are to describe the dispersion process, use Snell’s law for different colours, and link the effect to lens aberrations.
Lesson Structure:
  1. Do‑now (5’) – Students list everyday examples of colour separation (rainbow, soap bubbles). Teacher checks responses.
  2. Mini‑lecture (10’) – Review wave nature of light, refractive index, and Snell’s law; introduce dispersion concept with diagram.
  3. Demonstration (8’) – Show prism with white light; students observe colour order and record angles on worksheet.
  4. Guided analysis (12’) – Use provided table of refractive indices to calculate expected deviation for red and blue using Δθ ≈ (n_blue‑n_red)·sinA; discuss results.
  5. Group activity (10’) – Students predict how changing prism angle or material would affect the spectrum; share findings.
  6. Check for understanding (5’) – Quick quiz (exit ticket) with three short questions on definitions and calculations.
Conclusion:
Summarise that dispersion occurs because the refractive index varies with wavelength, causing each colour to bend differently and produce a spectrum. Re‑emphasise the link to chromatic aberration in lenses and how coatings mitigate the effect. For homework, students complete a worksheet calculating deviation for a different glass and write a brief explanation of why rainbows form.