Diffraction
Cambridge A-Level Physics (9702) · Unit 8: Superposition · 7 flashcards
Diffraction is topic 8.2 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 8 — Superposition , alongside Stationary waves, Interference and The diffraction grating. In one line: Diffraction is the spreading out of waves as they pass through an aperture or around an obstacle. The effect is most significant when the wavelength is comparable to the size of the aperture.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 7 flashcards — 1 definition and 6 key concepts — covering the precise wording mark schemes reward. Use the definition card to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Diffraction
Diffraction is the spreading out of waves as they pass through an aperture or around an obstacle. The effect is most significant when the wavelength is comparable to the size of the aperture.
What the Cambridge 9702 syllabus says
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Explain what diffraction means.
- Understand experiments that show diffraction (a ripple tank with water waves is one example), including, qualitatively, how the gap width compared with the wavelength affects it.
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Diffraction
- › Remember that diffraction spreads waves perpendicular to the long axis of the slit; a vertical slit produces a horizontal pattern.
Define diffraction.
Diffraction is the spreading out of waves as they pass through an aperture or around an obstacle. The effect is most significant when the wavelength is comparable to the size of the aperture.
Describe how the width of a gap affects the amount of diffraction.
The amount of diffraction is greatest when the gap width is approximately equal to the wavelength of the wave. As the gap width increases relative to the wavelength, the amount of diffraction decreases.
What happens to the diffraction pattern when the wavelength of the wave increases (gap size constant)?
Increasing the wavelength (for a constant gap size) leads to increased diffraction. The waves spread out more after passing through the gap.
Describe an experiment that demonstrates diffraction.
Water waves in a ripple tank can be used. Place a barrier with a narrow gap in the tank. When plane waves are directed towards the gap, they will diffract and spread out on the other side.
Explain how the wavelength of water waves relates to the size of the gap needed for significant diffraction in a ripple tank experiment.
For observable diffraction in a ripple tank, the wavelength of the water wave should be similar to, or larger than, the width of the gap. If the wavelength is significantly smaller, diffraction will be minimal.
Sketch the wave pattern after diffraction through a 'small' gap. Define 'small'.
Sketch: Semicircular wavefronts emanating from the gap. 'Small' means the gap width is approximately the same size or smaller than the wavelength of the wave.
Sketch the wave pattern after waves pass through a very wide gap (relative to their wavelength).
Sketch: The waves pass through the gap with minimal bending. The wavefronts remain mostly planar, with slight diffraction effects near the edges of the gap. Most of the wave continues straight.
Review the material
Read full revision notes on Diffraction — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 8 — Superposition
Diffraction sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Diffraction deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
How to study this Diffraction deck
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