The shape of the lesson
By the end of the lesson, learners can
- Tune the gap to the wavelength: Change the gap width and the wavelength and watch how much the waves spread beyond the gap.
- It is the ratio that matters: Whether a wave spreads a lot or a little comes down to comparing the gap with the wavelength.
- Spacing never changes. The wavefronts after the gap or edge must be exactly as far apart as before it. Diffraction does not change the wavelength.
- Wavefronts curve in the direction of travel. They bulge forward, away from the gap or edge, like ripples spreading outward, never back toward the source.
- At an edge, the wave wraps into the shadow. The edge acts like a new source. The wave bends around the corner into the region behind the obstacle, it does not leave a perfectly sharp shadow.
One picture every idea returns to
Diffraction is the spreading of waves through a gap or around an edge. The effect depends on the ratio of the gap width to the wavelength: spreading is strongest when the gap is comparable to the wavelength, and for a given gap, longer wavelengths diffract more.
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, When the gap matches the wave.. Stand behind a wall and you can still hear someone speaking, but you cannot see them. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Tune the gap to the wavelength | Change the gap width and the wavelength and watch how much the waves spread beyond the gap. Project tab 5 of the lab (Diffraction in depth) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: It is the ratio that matters | Whether a wave spreads a lot or a little comes down to comparing the gap with the wavelength. | Mix-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, 27 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 It is the ratio that matters, 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.
Mix-Pair-Share
Students mix around the room until you call pair; they pair with the nearest classmate, discuss the prompt, then mix again. Three prompts of rising difficulty, three different partners.
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 |
|---|---|
| It is the ratio of gap to wavelength that matters, not the absolute size of the gap. | A gap is only effectively narrow if it is small compared with the wavelength, so the same gap can diffract long waves strongly and short waves hardly at all. |
Diffraction in depth: worksheet
8 questions, 27 marks. Written in the style of the papers; no past paper question is reproduced.
A wave passes through a gap in a barrier.
(a) State the condition for the wave to diffract as strongly as possible. [1] (b) Describe what the diffraction pattern looks like when the gap is much wider than the wavelength. [2]The diagram shows plane water waves in a ripple tank approaching a gap. The gap width is approximately equal to the wavelength of the waves.
On a copy of the diagram, draw the wavefronts after they have passed through the gap.
The diagram shows plane water waves approaching the straight edge of a long obstacle. To the right of the edge the water is open; behind the obstacle is the shadow region.
On a copy of the diagram, draw the wavefronts after they have passed the edge. Show clearly what happens both in the open region and in the shadow region behind the obstacle.
As a water wave diffracts through a gap, state what happens to each of the following, and give a reason.
(a) its wavelength [1] (b) its speed [1] (c) its frequency [1]A person stands behind a tall, wide wall. They cannot see a car on the far side, but they can clearly hear its engine.
Explain this observation in terms of diffraction and wavelength.
A house sits in a valley, with a large hill between it and a distant transmitter. The residents can receive long-wave radio (wavelength about 1500 m) clearly, but cannot receive television signals (wavelength about 0.5 m).
Explain, in terms of diffraction, why the long-wave radio reaches the house but the television signal does not.
Waves of the same wavelength are sent through two different gaps in separate experiments. Gap A is much wider than the wavelength. Gap B is approximately equal to the wavelength.
For each gap, sketch the pattern of the wavefronts on the far side. Make clear how the two patterns differ.
A student draws the wavefronts spreading out from a narrow gap. In their drawing, the curved wavefronts get further and further apart as they move away from the gap.
State what is wrong with the drawing, and explain what the student has misunderstood.
Answer key and marking notes
(a) The gap width is approximately equal to the wavelength of the wave. ✓
(b) The waves pass through almost straight, staying as plane (straight) wavefronts, with only slight curving at the two edges of the gap. ✓✓
Strong diffraction needs gap ≈ wavelength. A wide gap gives weak diffraction.
Your drawing should show semicircular wavefronts spreading out from the gap, like ripples from a point source.
Equal spacing. Curving forward. Centred on the gap.Check with a ruler: the gaps between your curves must match the spacing of the incoming waves. If they grow wider, that is the most common and most penalised mistake.
- Open region: wavefronts stay straight and keep moving forward.
- Near the edge: they curl around the corner into the shadow.
- Spacing stays the same everywhere.
(a) Wavelength stays the same. ✓
(b) Speed stays the same. ✓
(c) Frequency stays the same. ✓
The wave stays in the same medium (same water), so none of its properties change. Diffraction changes only the shape and direction of the wavefronts, not the wave itself.
Contrast with refraction, where the medium DOES change, so speed and wavelength change. Diffraction keeps the wave in one medium, so nothing about the wave changes.
- Both sound and light diffract (bend) around the edges of the wall.
- The amount of diffraction depends on how the wavelength compares with the size of the obstacle.
- Sound has a long wavelength, comparable to the size of the wall, so it diffracts strongly and bends around into the shadow where the person stands. They hear it.
- Light has an extremely short wavelength, far smaller than the wall, so it barely diffracts. It travels almost straight and leaves a sharp shadow, so the person cannot see the car.
- The long-wave radio has a wavelength (about 1500 m) comparable to, or larger than, the size of the hill. So it diffracts strongly and bends around the hill into the valley.
- The television signal has a much shorter wavelength (about 0.5 m), far smaller than the hill. So it diffracts very little and travels almost straight, leaving the valley in the hill's shadow.
- Gap A (wide): waves pass through almost straight, with only slight curving at the edges.
- Gap B (equal): waves spread out into strong semicircular wavefronts, like a point source.
- In both, the spacing of the wavefronts is unchanged.
- The wavefronts should stay the same distance apart, not get further apart.
- The distance between wavefronts is the wavelength, and diffraction does not change the wavelength.
- By drawing them spreading apart, the student has wrongly shown the wavelength (and therefore the speed) increasing, which does not happen.
3 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 pass through a gap in a barrier. The diffraction (spreading) is greatest when:
- AThe gap is about the same size as the wavelength.
- BThe gap is very much larger than the wavelength.
- CThe wavelength is zero.
- DThe waves are travelling very fast.
Waves of different wavelengths pass through the same fixed gap. Which waves spread out the most?
- Athose with the longest wavelength
- Bthose with the shortest wavelength
- Cthose with the highest frequency
- Dthose with the smallest amplitude
In a ripple tank, water waves of one wavelength meet a barrier with two gaps: one narrow gap and one wide gap. Through which gap do the waves spread more?
- Athe narrow gap
- Bthe wide gap
- Cthey spread equally through both
- Dthey do not spread through either
Quiz key
Diffraction is greatest when the gap width is about equal to the wavelength. A gap much wider than the wavelength (B) lets the waves pass almost straight through with little spreading.
For a fixed gap, the longest wavelengths diffract the most because their wavelength is closest to (or larger than) the gap width. Shorter wavelengths (higher frequencies) spread the least.
The narrow gap is closer to the wavelength, so the waves diffract and spread more. Through the wide gap the waves continue almost straight with only slight spreading at the edges.
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.