The shape of the lesson
By the end of the lesson, learners can
- Make ripples misbehave: Send waves at a barrier, across a boundary, and through a gap, and watch them reflect, refract and diffract.
- Three behaviours, three causes: Each effect has a clear cause and a clear rule.
- Speed. Did the medium change? If yes, speed changes. This is the cause of everything else.
- Wavelength. It follows the speed, because v = fλ and frequency is fixed. Slower means shorter.
- Frequency. Set by the source. It never changes at a boundary. State this explicitly.
- Direction. Only bends if the wave meets the boundary at an angle. Head-on, no bend.
One picture every idea returns to
Waves reflect when they bounce off a barrier, with the angle of incidence equal to the angle of reflection. They refract when they change speed crossing a boundary, which bends them. They diffract when they spread out through a gap or around an edge, most strongly when the gap is about the size of the wavelength.
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, Bounce, bend, spread.. Send straight ripples across a tank and you can make them do three things: bounce off a barrier, bend as they cross into shallower water, and fan out as they squeeze through a gap. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Make ripples misbehave | Send waves at a barrier, across a boundary, and through a gap, and watch them reflect, refract and diffract. Project tab 4 of the lab (Reflection, refraction, diffraction) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Three behaviours, three causes | Each effect has a clear cause and a clear rule. | Think-Pair-Share |
| 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 (8 questions, 28 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 Three behaviours, three causes, 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.
Think-Pair-Share
Pose the question, give thirty seconds of silent thinking, then a minute to compare with a partner before a few pairs share with the class. The silent minute is the part that cannot be cut.
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 |
|---|---|
| When a wave refracts, its frequency does not change; only its speed and wavelength do. | And diffraction is most noticeable when the gap is about equal to the wavelength, not simply when the gap is small. |
Reflection, refraction and diffraction of waves: worksheet
8 questions, 28 marks. Written in the style of the papers; no past paper question is reproduced.
A plane water wave is reflected from a straight barrier in a ripple tank.
(a) State the relationship between the angle of incidence and the angle of reflection. [1] (b) State what happens to the speed, frequency and wavelength of the wave during reflection. [2]In a ripple tank, plane water waves travel from a region of deep water into a region of shallow water. The boundary between the regions is at an angle to the wavefronts.
Analyse what happens to the speed, wavelength, frequency and direction of the waves as they cross into the shallow water. Give a reason for each.
A student is confused. They write: "When a wave slows down as it enters a new medium, its frequency must drop, because it is moving more slowly."
Explain why the student is wrong. In your answer, make clear what actually happens to the frequency and the wavelength, and why.
The diagram shows plane water wavefronts travelling straight downward and crossing a boundary between two regions of a ripple tank. The wave meets the boundary head-on (the wavefronts are parallel to the boundary).
Two people stand on opposite sides of an open doorway, around a corner from each other, so that they cannot see one another. They find they can hear each other talking clearly, but cannot see each other at all.
Analyse this situation using the idea of diffraction. Explain why the sound reaches around the corner but the light does not.
The diagram shows plane water waves in a ripple tank approaching a gap in a barrier, and what they look like after passing through.
A wave crosses from one medium into another in which it travels more slowly. It meets the boundary along the normal (perpendicular to the boundary).
The wave slows down, yet it does not change direction. Explain how it is possible for the speed to change without the direction changing.
Copy and complete the table by writing "changes" or "stays the same" for the wavelength of a water wave during each behaviour.
(a) Reflection from a barrier [1] (b) Refraction into shallower water [1] (c) Diffraction through a gap [1]Answer key and marking notes
(a) The angle of incidence equals the angle of reflection (both measured from the normal). ✓
(b) Speed, frequency and wavelength all stay the same. Only the direction changes. ✓✓
Reflection is the "nothing changes except direction" behaviour. The medium has not changed, so there is no reason for speed or wavelength to change.
- Speed decreases. Water waves travel more slowly in shallower water.
- Wavelength decreases. Because v = fλ and frequency is fixed, a smaller v means a smaller λ.
- Frequency stays the same. It is set by the source (the vibrating bar), not by the medium.
- Direction bends toward the normal. Because the waves meet the boundary at an angle, one end of each wavefront slows before the other, twisting the wave toward the normal.
- Frequency is set by the source, which has not changed. The same number of wavefronts arrive at the boundary each second as leave it. So frequency stays constant.
- If frequency stayed the same but wavefronts piled up at the boundary, that would be impossible. So instead the wavelength shrinks.
- It is the wavelength that absorbs the change in speed, not the frequency: v = fλ, with f fixed, so a smaller v forces a smaller λ.
(a) Refraction. ✓
(b) Region Y is shallower. The wavefronts are closer together there, so the wavelength is shorter, which means the wave has slowed down, and water waves are slower in shallower water. ✓✓
(c) The wave meets the boundary head-on (wavefronts parallel to the boundary), so both ends of each wavefront slow at the same instant. With no twisting, there is no change in direction. ✓
Closer wavefronts = shorter wavelength = slower wave = shallower water. Follow that chain and part (b) is automatic.
- Both sound and light diffract (spread out) when they pass through the doorway.
- The amount of diffraction depends on how the wavelength compares with the width of the gap (the doorway).
- Sound has a long wavelength, comparable to the size of the doorway, so it diffracts strongly and spreads around the corner to the listener.
- Light has an extremely short wavelength, far smaller than the doorway, so it barely diffracts and continues almost in a straight line. It does not bend around the corner, so they cannot see each other.
(a) Diffraction. ✓
(b) The wavelength stays the same. Diffraction does not change wavelength, speed or frequency, only the shape of the wavefronts. ✓
(c) Make the gap narrower (closer to the wavelength of the wave). ✓
Part (b) is the trap. Diffraction changes the shape of the wavefronts but NOT the wavelength. Many students wrongly say the wavelength decreases.
- Refraction bends a wave only when one part of a wavefront enters the slower medium before another part, twisting the wavefront.
- Here the wave meets the boundary head-on, so the whole wavefront crosses at the same instant.
- Every part slows together, so there is no twisting and the direction stays the same. The speed (and wavelength) still change.
(a) Reflection → wavelength stays the same ✓
(b) Refraction → wavelength changes (decreases) ✓
(c) Diffraction → wavelength stays the same ✓
Only refraction changes the wavelength, because only refraction involves a change of speed. Reflection and diffraction keep the wave in the same medium, so the speed and wavelength are unchanged.
1 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.
Water waves slow down as they move from deep water into shallow water. What happens to their wavelength and frequency?
- AThe wavelength decreases and the frequency stays the same.
- BBoth the wavelength and the frequency decrease.
- CThe wavelength stays the same and the frequency decreases.
- DBoth increase.
Quiz key
The frequency is set by the source and does not change, so from v = fλ a lower speed means a shorter wavelength. Option B wrongly changes the frequency.
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.