| Lesson Plan |
| Grade: |
Date: 25/02/2026 |
| Subject: Physics |
| Lesson Topic: Know that the speed of electromagnetic waves in a vacuum is 3.0 × 10^8 m/s and is approximately the same in air |
Learning Objective/s:
- Describe why electromagnetic waves travel at the same speed in a vacuum and why the speed in air differs by less than 0.03 %.
- Explain and apply the relationship c = λ f to calculate frequency or wavelength of EM waves.
- Identify the main regions of the electromagnetic spectrum and match each with typical wavelength/frequency ranges and everyday uses.
- Solve exam‑style problems that require using the speed of light constant in calculations.
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Materials Needed:
- Projector and screen
- Printed EM‑spectrum handout
- Calculator worksheets
- Laser pointer for a brief demo
- Short video clip showing the speed of light
- Interactive simulation (e.g., PhET “Wave on a String” adapted for EM waves)
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Introduction:
Begin with the question, “What would it be like if a message could travel instantly from one side of the Earth to the other?” Students recall prior learning about wave speed and the formula v = fλ. Explain that today they will discover why that “instant” speed is the same for all electromagnetic waves in a vacuum and almost unchanged in air, and they will learn how to use it in calculations.
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Lesson Structure:
- Do‑now (5′): Quick worksheet – calculate frequency from a given wavelength using c = 3.0 × 10⁸ m s⁻¹.
- Mini‑lecture (10′): Present key concepts – EM waves need no medium, speed in vacuum, negligible difference in air, and c = λ f.
- Interactive simulation (8′): Students manipulate wavelength and frequency sliders to see c remain constant.
- Spectrum overview (7′): Show diagram, discuss each region’s typical λ and f, and link to real‑world applications.
- Guided practice (12′): Solve two exam‑style problems together – (a) speed of light in air, (b) wavelength of a 2.4 GHz microwave.
- Independent practice (10′): Worksheet with three mixed questions; teacher circulates for formative feedback.
- Check for understanding (5′): Exit ticket – one sentence explaining why the speed in air can be taken as c.
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Conclusion:
Summarise that all electromagnetic waves travel at c ≈ 3.0 × 10⁸ m s⁻¹ in vacuum and effectively the same in air, allowing us to use the simple relationship c = λ f for calculations across the spectrum. Collect the exit tickets and assign a short homework task: calculate the wavelength of a radio wave at 100 MHz and the frequency of an infrared wave with λ = 5 µm.
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