What is polarisation in the context of waves in Physics?

Polarisation refers to the orientation of oscillations in a transverse wave, such as light. In Physics, particularly at the Cambridge International A Levels, polarisation is studied to understand how waves can oscillate in different directions. For example, light waves can be polarised to oscillate in a single plane, which is useful in various applications like reducing glare in sunglasses and enhancing contrast in LCD screens.

How does polarisation occur in light waves?

Polarisation of light waves occurs when the waves are restricted to oscillate in a single plane. This can happen through reflection, refraction, or by passing light through a polarising filter. When unpolarised light, which has waves oscillating in multiple planes, passes through a polarising filter, only the waves oscillating in the direction of the filter's axis are allowed through, resulting in polarised light.

What are some practical applications of polarisation?

Polarisation has several practical applications in everyday life and technology. In the Cambridge International A Levels curriculum, students learn about its use in reducing glare with polarised sunglasses, enhancing image quality in photography, and in liquid crystal displays (LCDs). Additionally, polarisation is crucial in optical communication systems and in studying stress patterns in transparent materials through polarised light.

How can polarisation be demonstrated in a laboratory setting?

In a laboratory setting, polarisation can be demonstrated using polarising filters and light sources. A common experiment involves passing unpolarised light through a polarising filter and observing the intensity of light as it passes through a second filter, known as an analyser. By rotating the analyser, students can observe changes in light intensity, demonstrating the concept of polarisation and the effect of aligning the polarisation axes of the filters.

What is the difference between linear and circular polarisation?

Linear polarisation occurs when the electric field of a light wave oscillates in a single plane. In contrast, circular polarisation involves the electric field rotating in a circular motion as the wave propagates. This can be visualised as the tip of the electric field vector tracing out a circle. Circular polarisation is achieved by passing linearly polarised light through a quarter-wave plate, which introduces a phase shift between the orthogonal components of the light wave.

Why is "Using $\cos\theta$ instead of $\cos^2\theta$" a common mistake in Polarisation ?

Why it happens: Forgetting squaring. How to avoid it: Malus' law uses ^2. The relates AMPLITUDES; squaring gives INTENSITIES. Source: 9702 Examiner Reports 2022-2024

Why is "Saying sound can be polarised" a common mistake in Polarisation ?

Why it happens: Confusing wave types. How to avoid it: Only TRANSVERSE waves can be polarised. Sound (longitudinal) CANNOT. Source: 9702 Examiner Reports 2022-2024

WavesCambridge International A Levels Physics (9702)

Polarisation

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Polarisation Study Notes — Cambridge International A Level Physics 9702 (2025-2027 syllabus)

Plane polarisation. Malus' law for polariser. Only transverse waves can polarise.

What you’ll learn

Mapped to the Cambridge IGCSE 9702 syllabus (2025-2027).

7.5.1 — Define polarisation.
7.5.2 — Apply Malus' law.
7.5.3 — Distinguish polarisable waves.

Polarisation

Single-plane oscillation.

Plane-polarised wave. Oscillation confined to one plane containing the direction of travel.

Only transverse waves can be polarised. Longitudinal waves (sound) have oscillation along travel direction — no plane to confine.

Source of polarisation.

Polariser filter.
Reflection at certain angle.
Scattering (sky).

Cambridge tip. Polarisation demonstrates that light is TRANSVERSE.

Single-plane oscillation.
Transverse only.
Shows light is transverse.

Malus' law

$I = I_0 \cos^2\theta$ .

For polarised light through polariser at angle $\theta$ to polarisation direction: $I = I_0 \cos^2\theta$

Why squared? Amplitude scales as $\cos\theta$ (vector projection); intensity $\propto$ amplitude squared.

Special cases.

$\theta = 0°$ : $I = I_0$ (parallel — full transmission).
$\theta = 45°$ : $I = I_0/2$ .
$\theta = 90°$ : $I = 0$ (crossed — no transmission).

Unpolarised through one polariser. All angles equally present; average $\cos^2\theta = 1/2$ . So transmitted intensity = $I_0/2$ .

Cambridge tip. Remember unpolarised case is different — averages give 1/2.

A first polariser halves unpolarised light's intensity; a second polariser at angle θ then transmits a further factor of cos²θ (Malus's law).

$I = I_0 \cos^2\theta$ .
Crossed → 0.
Unpolarised through one polariser → $I_0/2$ .

See the full worked example for polarisation →

How it’s examined

Polarisation on Paper 1 or 2 typically 5-6 marks. Most-tested: Malus' law calculation (5-6 marks), crossed polarisers (5 marks).

Sources: Cambridge International A Level Physics 9702 syllabus (2025-2027); 9702 Examiner Reports 2022-2024; 9702/22 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-11.

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Step-by-step worked examples — Polarisation

Step-by-step solutions to past-paper-style questions on polarisation , written exactly the way a tutor would explain them at the board.

1Malus' law (6 marks)
Extended• Adapted from 9702/22 May/Jun 2024• Malus
Question
Polarised light of intensity $I_0 = 8.0$ W/m² passes through a polariser at $30°$ to the polarisation direction. Find transmitted intensity. (6 marks)
Step-by-step solution
1. Step 1
  Malus' law: $I = I_0 \cos^2\theta$ .
  $I = 8.0 \times \cos^2 30° = 8.0 \times 0.75 = 6.0 \text{ W/m}^2$
Answer
$I = 6.0$ W/m².
2Unpolarised through one polariser (5 marks)
Extended• unpolarised
Question
Unpolarised light intensity $I_0$ passes through a single polariser. State transmitted intensity. (5 marks)
Step-by-step solution
1. Step 1
  Unpolarised light has all polarisation directions equally. Polariser passes half.
  $I = \dfrac{I_0}{2}$
Answer
$I_0/2$ .
3Crossed polarisers (5 marks)
Extended• crossed
Question
Light passes through two polarisers with axes at $90°$ to each other. What is the transmitted intensity? (5 marks)
Step-by-step solution
1. Step 1
  Malus' law with $\theta = 90°$ . $\cos^2 90° = 0$ .
2. Step 2
  No light transmitted.
Answer
$I = 0$ . Crossed polarisers block all light.

Key Formulae — Polarisation

The formulae you need to memorise for polarisation on the Cambridge International A Level 9702 paper, with every variable defined in plain English and a note on when to use it.

Malus' law
$I = I_0 \cos^2\theta$
$\theta$
angle between polarisation direction of light and polariser axis
When to use
Transmitted intensity of polarised light through polariser.

Key Definitions and Keywords — Polarisation

Definitions to memorise and the exact keywords mark schemes credit for polarisation answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9702 sitting.

Polarisation
Examiner keyword
Oscillation of a transverse wave restricted to a single plane.
Plane-polarised
Examiner keyword
Wave with oscillations in a single plane containing the direction of travel.
Polariser
Examiner keyword
Filter that transmits only one polarisation direction.

Common Mistakes and Misconceptions — Polarisation

The traps other students keep falling into on polarisation questions — taken from recent Cambridge International A Level 9702 examiner reports and mark schemes — and how to avoid them.

✕Using $\cos\theta$ instead of $\cos^2\theta$
9702 Examiner Reports 2022-2024
Why it happens
Forgetting squaring.
How to avoid it
Malus' law uses $\cos^2\theta$ . The $\cos$ relates AMPLITUDES; squaring gives INTENSITIES.
✕Saying sound can be polarised
9702 Examiner Reports 2022-2024
Why it happens
Confusing wave types.
How to avoid it
Only TRANSVERSE waves can be polarised. Sound (longitudinal) CANNOT.

Practice questions

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Polarisation — frequently asked questions

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Polarisation

Short Study Notes in the form of Slides

Detailed Notes

What you’ll learn

How it’s examined

1Malus' law (6 marks)

2Unpolarised through one polariser (5 marks)

3Crossed polarisers (5 marks)

Malus' law

Polarisation

Plane-polarised

Polariser

✕Using cos⁡θ\cos\thetacosθ instead of cos⁡2θ\cos^2\thetacos2θ

✕Saying sound can be polarised

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Polarisation — frequently asked questions

✕Using $\cos\theta$ instead of $\cos^2\theta$