Polarisation
Cambridge A-Level Physics (9702) · Unit 7: Waves · 6 flashcards
Polarisation is topic 7.5 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 7 — Waves , alongside Progressive waves, Transverse and longitudinal waves and Doppler effect for sound waves. In one line: Polarisation is a phenomenon exhibited by transverse waves only. Longitudinal waves cannot be polarised because their oscillations are parallel to the direction of propagation.
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 6 flashcards — 3 definitions, 2 key concepts and 1 calculation — covering the precise wording mark schemes reward. Use the 3 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.
What type of wave exhibits polarisation
Polarisation is a phenomenon exhibited by transverse waves only. Longitudinal waves cannot be polarised because their oscillations are parallel to the direction of propagation.
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 that polarisation happens only with transverse waves.
- Recall and use Malus's law, I = I₀ cos²θ, to find the intensity of a plane-polarised electromagnetic wave after it passes through one or more polarising filters. Finding how a filter affects an unpolarised wave's intensity is not required.
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 Polarisation
- › Memorize Malus’s Law as I = I_0 cos^2 θ and ensure you use the correct angle between the polariser and analyser.
What type of wave exhibits polarisation?
Polarisation is a phenomenon exhibited by transverse waves only. Longitudinal waves cannot be polarised because their oscillations are parallel to the direction of propagation.
Define polarisation in the context of transverse waves.
Polarisation refers to the restriction of the oscillations of a transverse wave to a single plane. This plane contains the direction of propagation.
State Malus's Law.
Malus's Law states that the intensity (I) of plane-polarised light after passing through a polariser is given by I = I₀cos²θ, where I₀ is the initial intensity and θ is the angle between the polariser's transmission axis and the plane of polarisation of the light.
A polarising filter is rotated from 0° to 90° relative to the plane of polarised light. How does the transmitted intensity change?
As the angle increases from 0° to 90°, the transmitted intensity decreases. At 0°, the intensity is maximum (I₀), and at 90°, the intensity is zero.
Plane-polarised light of intensity 20 W/m² passes through a polarising filter oriented at 30° to the plane of polarisation. What is the intensity of the transmitted light?
Using Malus's Law: I = I₀cos²θ = 20 W/m² * cos²(30°) = 20 W/m² * (√3/2)² = 20 * (3/4) = 15 W/m².
Unpolarised light is incident on two polarising filters. The first filter has its transmission axis vertical, and the second filter has its transmission axis at 45° to the vertical. Describe the light after passing through both filters, with explanation.
The first filter polarizes the light vertically. Then Malus's Law tells us the second reduces the intensity by cos²(45°) = (√2/2)² = 1/2. The light is plane polarised at 45° to vertical, and has intensity I = I₀ * 1/2, where I₀ is the intensity of the polarised light after passing through the first filter.
Review the material
Read full revision notes on Polarisation — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 7 — Waves
Polarisation 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 Polarisation 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 Polarisation deck
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