| Lesson Plan |
| Grade: |
Date: 25/02/2026 |
| Subject: Physics |
| Lesson Topic: Describe internal reflection and total internal reflection using both experimental and everyday examples |
Learning Objective/s:
- Describe internal reflection and differentiate it from total internal reflection.
- Calculate the critical angle for given media using refractive indices.
- Predict when total internal reflection occurs in experimental and everyday contexts.
- Explain how optical fibres and diamonds use total internal reflection.
- Conduct a simple experiment to observe the critical angle and compare with theoretical values.
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Materials Needed:
- Transparent glass block or acrylic prism
- Ray box with a narrow light beam
- Protractor or rotating stage
- White screen to capture emergent ray
- Ruler or measuring tape
- Worksheet with calculation and reflection questions
- Projector and slide set (optional)
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Introduction:
Begin with a quick demonstration of a laser beam striking a water surface, prompting students to guess why a bright line appears. Recall the law of reflection and Snell’s law from previous lessons as the foundation for today’s topic. Explain that by the end of the lesson they will be able to identify and calculate the critical angle and describe real‑world applications such as fibre optics.
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Lesson Structure:
- Do‑now (5'): Students answer a short question on why a mirror reflects light and write responses on a sticky note.
- Mini‑lecture (10'): Review internal reflection, introduce the critical‑angle formula, and show a slide with a table of common materials.
- Guided demonstration (15'): Set up the glass block, ray box, and protractor; increase the incidence angle, observe the disappearance of the emergent ray, and measure the critical angle.
- Calculation activity (10'): Pupils use provided refractive indices to compute theoretical critical angles and compare them with their measurements.
- Everyday examples discussion (10'): In groups, analyse how optical fibres, diamonds, and mirages rely on total internal reflection; each group shares a brief explanation.
- Check for understanding (5'): Exit ticket where students write the condition for total internal reflection and one real‑world example.
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Conclusion:
Summarise that internal reflection occurs when light moves from a denser to a rarer medium and that total internal reflection happens only when the incidence angle exceeds the critical angle. Ask students to complete an exit ticket stating the critical‑angle condition and one application, which will be reviewed next class. Assign homework to research another technology that uses total internal reflection and prepare a short description.
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