Lesson Plan

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Lesson Plan
Grade: Date: 17/01/2026
Subject: Physics
Lesson Topic: understand the appearance and formation of emission and absorption line spectra
Learning Objective/s:
  • Describe quantised energy levels and the role of quantum numbers in atoms.
  • Explain how electron transitions generate emission and absorption lines.
  • Apply the Rydberg formula to calculate wavelengths of the Balmer series.
  • Analyse factors that influence line intensity such as population, transition probability and selection rules.
  • Interpret spectral series to identify elements from their line spectra.
Materials Needed:
  • Projector and slide deck
  • Gas discharge tube with spectroscope (or online simulation)
  • Worksheet with Rydberg‑formula problems
  • Laser pointer for demonstration of stimulated emission
  • Whiteboard and markers
  • Colored filters (optional for visualising absorption)
 Introduction:

Begin with a striking image of a rainbow‑like emission spectrum to capture interest. Ask students what they recall about energy levels from the previous lesson and link that knowledge to today’s focus. State that by the end of the class they will be able to predict and calculate the colours of spectral lines and explain why they appear bright or dark.
Lesson Structure:
  1. Do‑now (5') – Quick quiz on energy levels and photon energy (hν = ΔE).
  2. Mini‑lecture (10') – Review quantised levels, introduce ΔE = hc/λ and the Rydberg formula using slides.
  3. Demonstration (8') – Show an emission spectrum from a discharge tube; point out bright lines and discuss why they appear.
  4. Guided activity (12') – Students work in pairs to calculate Balmer wavelengths (Hα, Hβ, …) and fill a table on the worksheet.
  5. Group discussion (10') – Analyse factors affecting line intensity (population, A‑coefficients, selection rules) and compare emission vs. absorption examples.
  6. Check for understanding (5') – Exit ticket: write one key difference between emission and absorption spectra and one real‑world application.
Conclusion:

Summarise how electron transitions create distinct spectral lines and why each element has a unique pattern. Collect exit tickets to gauge understanding and assign a short homework: use the Rydberg formula to predict the wavelength of the next Balmer line (n = 7 → 2).