The shape of the lesson
By the end of the lesson, learners can
- Narrow the gap, widen the spread: Change the gap width relative to the wavelength and watch the diffracted wavefronts spread.
- Gap versus wavelength: The ratio of gap width to wavelength controls the spreading.
- Why sound bends round a door: Everyday diffraction comes from matching wavelengths to gap sizes.
- Diffraction: spreading at gaps and edges.
- Greatest: gap comparable to λ.
- All waves: sound, light, water.
One picture every idea returns to
Diffraction is the spreading of a wave as it passes through a gap or around the edge of an obstacle. The spreading is greatest when the gap width is comparable to the wavelength. A wide gap (much larger than the wavelength) gives little spreading; a gap close to one wavelength gives strong spreading. Diffraction is a property of all waves.
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, Waves bend round edges.. Waves do not only travel in straight lines: passing through a gap or round an obstacle they spread out. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Narrow the gap, widen the spread | Change the gap width relative to the wavelength and watch the diffracted wavefronts spread. Project tab 3 of the lab (Diffraction) and let a student drive it while the class predicts. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Gap versus wavelength | The ratio of gap width to wavelength controls the spreading. | Pairs |
| 21 to 30 min | Teach and check: Why sound bends round a door | Everyday diffraction comes from matching wavelengths to gap sizes. | 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 (6 questions, 12 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 Why sound bends round a door, 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 |
|---|---|
| Diffraction does not change the wavelength, frequency or speed of the wave; it only changes its direction of spread. | The spreading is greatest when the gap is comparable to the wavelength, not when it is far smaller or far larger. And diffraction happens for all waves, which is why both sound and light diffract, just by different amounts. |
Diffraction: worksheet
6 questions, 12 marks. Written in the style of the papers; no past paper question is reproduced.
Define diffraction.
State the condition on the gap width for diffraction to be most pronounced.
Explain why light usually casts sharp shadows while sound bends around corners.
State the effect, if any, of diffraction on the wavelength, frequency and speed of a wave.
In a ripple tank, describe what happens to the diffracted waves as the gap is gradually narrowed toward one wavelength.
Diffraction is a property of all waves. Name three different types of wave that diffract.
Answer key and marking notes
Diffraction is the spreading of a wave as it passes through a gap or travels around the edge of an obstacle ✓
The gap width should be about the same as the wavelength of the wave ✓
- Light has a very short wavelength compared with everyday gaps, so it diffracts very little and casts sharp shadows ✓
- Sound has a much longer wavelength, comparable to doorways, so it diffracts strongly and spreads around corners ✓
- Diffraction changes only the direction in which the wave spreads ✓
- The wavelength, frequency and speed are all unchanged ✓
- The waves spread out more and more ✓
- until, at a gap of about one wavelength, they spread into almost semicircular wavefronts ✓
- For example: sound waves, light (electromagnetic) waves and water waves ✓
- (microwaves and other waves also diffract) ✓
12 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.
Diffraction is the:
- Aspreading of waves at a gap or edge
- Bspeeding up of waves
- Creflection of waves
- Dsplitting into colours
Diffraction at a gap is greatest when the gap is:
- Amuch wider than the wavelength
- Bmuch narrower than a wavelength
- Cabout one wavelength wide
- Dexactly zero
A wide gap (many wavelengths) diffracts a wave:
- Aa great deal
- Bonly a little
- Cnot at all, it reflects
- Dinto colours
Diffraction occurs for:
- Alight only
- Bsound only
- Cwater waves only
- Dall types of wave
Sound bends around a doorway more than light because sound has a:
- Amuch longer wavelength
- Bhigher speed
- Clarger amplitude
- Dlower frequency that does not matter
Diffraction changes a wave\’s direction of spread but not its:
- Aamplitude only
- Bphase only
- Cwavelength, frequency or speed
- Dtype
Water waves of wavelength 3 cm spread most through a gap of about:
- A30 cm
- B3 cm
- C3 m
- D0.3 mm
As a gap is narrowed toward the wavelength, the diffracted wavefronts become:
- Aflatter
- Bstationary
- Cfaster
- Dmore nearly semicircular
Diffraction provides evidence that something behaves as a:
- Awave
- Bparticle
- Cgas
- Dfield only
Radio waves are received behind hills more easily than light reaches there because radio waves:
- Atravel faster
- Bare longitudinal
- Chave long wavelengths and diffract around the hill
- Dcarry more energy
When a wave just passes the edge of an obstacle, it:
- Astops at the edge
- Bbends slightly into the shadow region
- Creflects fully
- Dchanges frequency
Light passing a very narrow single slit produces:
- Aa sharp single dot
- Bno light
- Ca rainbow only
- Da spread-out diffraction pattern
Quiz key
Diffraction is wave spreading at gaps and edges.
Maximum spreading occurs when the gap is comparable to λ.
A wide gap gives little spreading.
All waves diffract.
Sound wavelengths are comparable to the doorway, so they diffract strongly.
Diffraction leaves wavelength, frequency and speed unchanged.
Greatest spread when the gap is about one wavelength, 3 cm.
Strong diffraction at a narrow gap gives nearly semicircular wavefronts.
Diffraction is a characteristic wave behaviour.
Long radio wavelengths diffract around large obstacles.
Diffraction bends waves a little into the geometric shadow.
A narrow slit diffracts light into a spread pattern.
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.