• Quick retrieval from earlier learning (Colour, prisms and refraction; The law of reflection: calculate angles). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. A ray of light hits a flat mirror with an angle of incidence of 25°. What is the angle of reflection, in degrees? → 25 °Prompt: Both angles are measured from the normal.
2. A pencil in a glass of water looks bent. Why? (Light refracts (changes direction) as it passes from water to air / Light reflects off the water / The water bends the pencil / The glass is curved) → Light refracts (changes direction) as it passes from water to airPrompt: Light changes speed when it enters a new material.
3. A ray of light hits a flat mirror with an angle of incidence of 45°. What is the angle of reflection, in degrees? → 45 °Prompt: Both angles are measured from the normal.

• Ask the class to make a wave with a rope or slinky and describe what moves and what doesn't.
• Share the objective: Refraction: refractive index, Snell's law and critical angle. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: A ray of light enters a material from air with an angle of incidence of 60°. The angle of refraction is 33°. What is the refractive index of the material? (n = sin i ÷ sin r; give 2 decimal places)
• Step 1: sin 60° = 0.866, sin 33° = 0.5446
• Step 2: 0.866 ÷ 0.5446 = 1.59
• Step 3: So the answer is: 1.59
• Answer: 1.59
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. A ray of light enters a material from air with an angle of incidence of 70°. The angle of refraction is 36°. What is the refractive index of the material? (n = sin i ÷ sin r; give 2 decimal places) → 1.6Prompt: Use your calculator in degree mode.
2. Light passes from air into water (n = 1.33) with an angle of incidence of 50°. What is the angle of refraction, in degrees? (Give 1 decimal place.) → 35.2 °Prompt: sin r = sin i ÷ n

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 8 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. What is the critical angle for light going from water (n = 1.33) into air, in degrees? (sin c = 1 ÷ n; give 1 decimal place) → 48.8 °Prompt: sin c = 1 ÷ n
2. A ray of light enters a material from air with an angle of incidence of 60°. The angle of refraction is 35°. What is the refractive index of the material? (n = sin i ÷ sin r; give 2 decimal places) → 1.51Prompt: Use your calculator in degree mode.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-12.