Interference
Cambridge A-Level Physics (9702) · Unit 8: Superposition · 7 flashcards
Interference is topic 8.3 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 8 — Superposition , alongside Stationary waves, Diffraction and The diffraction grating. In one line: Interference is the superposition of two or more waves, resulting in a new wave pattern. This pattern can exhibit constructive interference (increased amplitude) or destructive interference (decreased amplitude).
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 — 2 definitions, 3 key concepts and 2 calculations — covering the precise wording mark schemes reward. Use the 2 definition cards 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.
Interference
Interference is the superposition of two or more waves, resulting in a new wave pattern. This pattern can exhibit constructive interference (increased amplitude) or destructive interference (decreased amplitude).
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).
- Understand what the terms coherence and interference mean.
- Understand experiments that produce interference from two coherent sources, covering all four of: microwaves, light, sound, and water waves in a ripple tank.
- Understand which conditions must be met for interference fringes from two sources to be seen.
- Recall λ = ax / D and use it for double-slit interference of light.
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 Interference
- › Apply the condition for destructive interference: path difference equals (n + 1/2)λ. For a second-order dark fringe, the path difference is 3/2λ.
- › In double-slit explanations, always state that waves must diffract at the slits to overlap and interfere.
Define interference.
Interference is the superposition of two or more waves, resulting in a new wave pattern. This pattern can exhibit constructive interference (increased amplitude) or destructive interference (decreased amplitude).
What is the definition of coherence in the context of waves?
Coherence describes waves with a constant phase relationship and the same frequency/wavelength. Coherent waves are necessary for observable and stable interference patterns.
Describe an experiment demonstrating two-source interference using light.
Young's double-slit experiment demonstrates interference. Coherent light passes through two narrow slits, creating an interference pattern of bright and dark fringes on a screen due to superposition.
What conditions are required to observe clear two-source interference fringes?
The two sources must be coherent (constant phase difference, so the same frequency). For clear dark fringes the waves should have similar amplitudes (and, for light, the same polarisation). The slit separation must be small compared with the slit-to-screen distance so the fringes are far enough apart to see.
State the equation for double-slit interference and define each term.
λ = ax / D, where λ is the wavelength of light, a is the slit separation, x is the fringe separation, and D is the distance from the slits to the screen.
If the slit separation in a double-slit experiment is doubled, what happens to the fringe separation?
Since λ = ax / D, if 'a' is doubled, 'x' will halve. Therefore, the fringe separation (x) decreases by a factor of two.
Explain why monochromatic light is preferred in double slit experiments.
Monochromatic light has a single, well-defined wavelength, which produces a clearer and more easily measurable interference pattern. Polychromatic light creates overlapping patterns, blurring the fringes.
Review the material
Read full revision notes on Interference — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 8 — Superposition
Interference 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 Interference 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.
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