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Science lesson plan · 12th grade (Year 13 in England) · 30 minutes

Refraction: refractive index, Snell's law and critical angle

Light, sound and waves

CCSS NGSS.HS-PS4-1CCSS NGSS.HS-PS4-3England: GCSE Physics: Waves (refraction)England: A level Physics: Refraction (extension)

Objective and success criteria

Students will: Refraction: refractive index, Snell's law and critical angle.

  • I can answer questions on today's skill on my own, without help.
  • I can explain each step and say why it works.
  • I can check my answer and spot a mistake.

Before and after

Needs first: Colour, prisms and refraction; The law of reflection: calculate angles

Leads to: the next skill in this area.

Materials: Slinky or rope, Mirrors and torches, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).

Warm-up · 4 min

  • 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. Why does a red apple look red in white light? (It makes red light / It absorbs red light / It reflects red light and absorbs the other colours / It reflects all colours) → It reflects red light and absorbs the other coloursPrompt: We see the colours that bounce off an object.
  3. A ray of light hits a flat mirror with an angle of incidence of 30°. What is the angle of reflection, in degrees? → 30 °Prompt: Both angles are measured from the normal.

Hook · 3 min

  • 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.

I do: model it · 7 min

  • 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.

We do: guided practice · 6 min

  • 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 40°. The angle of refraction is 25°. What is the refractive index of the material? (n = sin i ÷ sin r; give 2 decimal places) → 1.52Prompt: Use your calculator in degree mode.
  2. Light passes from air into diamond (n = 2.42) with an angle of incidence of 20°. What is the angle of refraction, in degrees? (Give 1 decimal place.) → 8.1 °Prompt: sin r = sin i ÷ n

You do: independent practice · 7 min

  • 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 6 questions are on the student sheet below, with answers in the key.

Exit ticket · 3 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. A ray of light enters a material from air with an angle of incidence of 50°. The angle of refraction is 29°. What is the refractive index of the material? (n = sin i ÷ sin r; give 2 decimal places) → 1.58Prompt: Use your calculator in degree mode.
  2. What is the critical angle for light going from glass (n = 1.5) into air, in degrees? (sin c = 1 ÷ n; give 1 decimal place) → 41.8 °Prompt: sin c = 1 ÷ n

Watch out for

  • Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
  • Remembering facts without the reason: ask "why?" and "what is the evidence?"

Support and stretch

Support

  • Review first: Colour, prisms and refraction.
  • Use concrete materials: slinky or rope.
  • Give the first step of each question, or a partly worked example to finish.
  • Pair with a confident partner for the guided practice.

Stretch

  • Explain a common mistake in writing and how to fix it.
  • Write a word problem that uses today's skill, then solve a partner's.

Homework and more practice

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

Free lesson plan from eurekami.com/lesson-plan-builder

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Refraction: refractive index, Snell's law and critical angle

Name

Date

Score/ 6

  1. Question 1

    A ray of light enters a material from air with an angle of incidence of 60°. The angle of refraction is 21°. What is the refractive index of the material? (n = sin i ÷ sin r; give 2 decimal places)

    Answer

  2. Question 2

    What is the critical angle for light going from an optical fibre core (n = 1.46) into air, in degrees? (sin c = 1 ÷ n; give 1 decimal place)

    Answer°

  3. Question 3

    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.)

    Answer°

  4. Question 4

    A ray of light enters a material from air with an angle of incidence of 60°. The angle of refraction is 41°. What is the refractive index of the material? (n = sin i ÷ sin r; give 2 decimal places)

    Answer

  5. Question 5

    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)

    Answer°

  6. Question 6

    Light passes from air into glass (n = 1.5) with an angle of incidence of 40°. What is the angle of refraction, in degrees? (Give 1 decimal place.)

    Answer°

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Answer key: Refraction: refractive index, Snell's law and critical angle

  1. 1. 2.42

  2. 2. 43.2 °

  3. 3. 35.2 °

  4. 4. 1.32

  5. 5. 48.8 °

  6. 6. 25.4 °