The shape of the lesson
By the end of the lesson, learners can
- Send a ray into the glass: Change the angle of the incoming ray and watch how it bends as it enters and leaves the block.
- Which way it bends: The direction of bending depends on whether the light is speeding up or slowing down.
- Cause is speed. Refraction happens because light changes speed at the boundary. Always start there.
- Bending rule. Into denser, bend toward the normal. Into less dense, bend away from the normal.
- Say "optically dense". Never "heavier" or "more dense". And remember frequency does not change.
One picture every idea returns to
Light changes speed when it crosses between materials of different optical density, and that change of speed bends the ray. Entering a denser material it slows and bends towards the normal; leaving it speeds up and bends away. A ray along the normal does not bend.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Bending at the boundary.. Push a straw into a glass of water and it looks broken at the surface. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Send a ray into the glass | Change the angle of the incoming ray and watch how it bends as it enters and leaves the block. Project tab 2 of the lab (Refraction) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Which way it bends | The direction of bending depends on whether the light is speeding up or slowing down. | Corners |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (7 questions, 20 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Which way it bends, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Corners
Put one answer or viewpoint in each corner of the room. Students move to the corner they agree with, justify the choice to a partner there, then hear one voice from each corner before the reveal.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| A ray travelling along the normal does not bend, even though it does change speed. | And refraction is caused by a change of speed: the frequency of the light stays the same throughout. |
Refraction of light: worksheet
7 questions, 20 marks. Written in the style of the papers; no past paper question is reproduced.
Define refraction, and state what causes it.
A ray of light passes from air into a glass block, and later leaves the glass block back into air.
(a) State which way the ray bends as it enters the glass, and explain why. [2] (b) State which way it bends as it leaves the glass, and explain why. [2]A student writes: "Light slows down in glass because glass is heavier than air." Explain why this answer would not earn the explanation mark, and give a correct version.
When light passes from air into glass, state what happens to each of the following: its speed, its wavelength, and its frequency.
A ray of light travels along the normal as it meets the flat surface of a glass block (angle of incidence = 0°).
(a) Does the ray change direction? Explain. [2] (b) Does the ray change speed? [1]The diagram shows a ray of light in air striking the flat surface of a glass block. The normal is drawn at the point where the ray meets the surface.
A straight straw standing in a glass of water appears to be bent at the point where it enters the water. Explain this observation in terms of refraction.
Answer key and marking notes
- Refraction is the change in direction of light as it passes from one transparent medium into another. ✓
- It is caused by a change in the speed of light at the boundary between the two media. ✓
(a) Toward the normal. Glass is optically denser than air, so the light slows down, which bends it toward the normal. ✓✓
(b) Away from the normal. Going back into air the light speeds up, which bends it away from the normal. ✓✓
Always tie the bending direction to the change in speed. Slower → toward. Faster → away.
- "Heavier" describes mass density, which is not the reason light slows. The examiner wants the optical property, not the weight.
- Correct version: light slows in glass because glass is optically denser than air. ✓
- Speed: decreases (light is slower in glass). ✓
- Wavelength: decreases (because speed falls and frequency is unchanged, from v = f λ). ✓
- Frequency: stays the same. ✓
(a) No. At 0° incidence the whole wavefront reaches the boundary at the same instant, so there is nothing to pivot the ray. It passes straight through. ✓✓
(b) Yes. It still slows down in the glass, even though its direction is unchanged. ✓
A favourite trap: students assume "refraction" must mean bending. Along the normal, the speed still changes but the direction does not.
- The refracted ray bends toward the normal, so r is smaller than i.
(a) Refracted ray drawn bending toward the normal, steeper than the incident ray. ✓✓
(b) The angle of refraction is smaller than the angle of incidence. ✓
Use a ruler. The refracted ray is a single straight line with an arrow, bending toward the normal.
- Light from the underwater part of the straw travels from water into air and refracts (bends away from the normal) as it leaves the surface.
- Your eye traces these rays back in straight lines, so the underwater part appears to be in a different place from where it really is.
- The two parts of the straw seem misaligned, so the straw looks bent at the surface. ✓✓✓
2 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
A ray of light passes from air into a glass block. How does it bend, and why?
- AToward the normal, because the light slows down in glass.
- BAway from the normal, because the light slows down in glass.
- CToward the normal, because the light speeds up in glass.
- DIt does not bend at all.
When light passes from air into water, which of its properties stays the same?
- Aits frequency
- Bits speed
- Cits wavelength
- Dits direction
Quiz key
Entering the denser glass, light slows down and bends toward the normal. Option B has the direction wrong; C has the speed change wrong, since light does not speed up entering glass.
The frequency is unchanged on refraction. The speed and wavelength both decrease in water, and the direction changes (it bends toward the normal).
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.