Lesson Plan

• Recall the definition of refractive index and the form of Snell’s law for air.
• Explain how to measure angles of incidence and refraction from the normal.
• Apply n = sin i / sin r to calculate a refractive index or to find an unknown angle.
• Identify and correct common mistakes when using the equation.

• Projector and screen
• Whiteboard and markers
• Protractor set (or digital protractor app)
• Scientific calculators (one per pair)
• Printed worksheet with practice questions
• Laser pointer and glass slab (demonstration)
• Snell’s law reference sheet

Begin with the everyday example of a straw appearing bent in a glass of water to spark curiosity about refraction. Review students’ prior knowledge of angles and the normal line. State that by the end of the lesson they will be able to calculate refractive indices and solve for unknown angles using n = sin i / sin r.

1. Do‑now (5'): Quick quiz on identifying angles of incidence and refraction.
2. Mini‑lecture (10'): Explain refraction, introduce the simplified equation n = sin i / sin r, and show the link to Snell’s law.
3. Demonstration (8'): Use a laser pointer and glass slab; students measure i and r with a protractor.
4. Guided practice (12'): Work through the provided example (i = 30°, r = 19°) and then solve practice questions in pairs.
5. Check for understanding (5'): Clicker quiz – calculate n or an unknown angle.
6. Summary & exit ticket (5'): Students write one new calculation on a sticky note; collect for review.

Recap the step‑by‑step process: measure angles, compute sines, divide to find n, and rearrange when needed. Ask students to submit an exit ticket showing a complete calculation. For homework, complete the remaining practice questions on the worksheet and bring any uncertainties to the next class.