Polarisation of Transverse Waves Revision Notes | A Level AQA Physics

1. Why Polarisation Matters

Here is a question that sounds deceptively simple: how do we know light is a transverse wave and not a longitudinal one? The answer lies in a phenomenon called polarisation — something that can only happen to transverse waves, and which has practical consequences everywhere from your sunglasses to the TV aerial on your roof.

Polarisation is one of those topics where the concept clicks quickly once you have the right mental picture. The challenge is making sure you can then use that picture to answer exam questions precisely — describing, explaining, and evaluating with the right language.

2. Transverse Waves and the Direction of Oscillation

Picture a transverse wave travelling horizontally away from you. The oscillations — the back-and-forth vibrations of the wave — are perpendicular to that direction of travel. For a wave on a rope, those oscillations might be up-and-down, or side-to-side, or at any angle in between. The key point is that they are always at right angles to the direction of energy transfer.

Now compare that with a longitudinal wave, such as sound. In a longitudinal wave, the oscillations are parallel to the direction of travel — compressions and rarefactions moving back and forth along the same line as the wave propagates. There is no concept of a perpendicular oscillation direction in a longitudinal wave, and that distinction is exactly why polarisation is only possible for transverse waves.

Think of unpolarised light as a bundle of waves, each vibrating in a different plane — some up-and-down, some side-to-side, some at 45°, and every other angle. All of these planes are perpendicular to the direction the light travels, but they are all present at once. A polarised wave, by contrast, has oscillations confined to a single plane.

3. Polarisation as Evidence for the Transverse Nature of Waves

This is the core specification point: polarisation is evidence that a wave is transverse. The logic runs like this — if you can polarise a wave, it must have oscillations in more than one possible perpendicular plane, which means it must be transverse. If a wave cannot be polarised, it is longitudinal.

Light can be polarised. This is direct experimental evidence that light is a transverse wave. Sound cannot be polarised — no matter what you do, you cannot restrict the oscillations of a sound wave to a single plane, because those oscillations are already along the direction of travel. This is direct evidence that sound is longitudinal.

When you pass unpolarised light through a polarising filter, the transmitted intensity drops to roughly half. The filter absorbs all components of the oscillation except those aligned with its transmission axis. The light that emerges is now plane-polarised — oscillating in one plane only. If you then place a second polarising filter (called the analyser) in the path and rotate it, you observe the transmitted intensity varying. When the two filters are aligned (transmission axes parallel), maximum light passes through. When they are perpendicular — called the crossed polarisers arrangement — virtually no light passes through. This behaviour is impossible to explain if light were longitudinal, and it is one of the most compelling demonstrations in wave physics.

4. Polaroid Material: How It Works

Polaroid is a synthetic material made from long-chain polymer molecules aligned in parallel rows. These chains absorb the component of the electric field oscillation that runs along their length, while transmitting the perpendicular component. The result is that only light oscillating in one specific plane — the transmission axis of the Polaroid — passes through.

When unpolarised light hits a Polaroid filter, the intensity of the transmitted light is approximately half the incident intensity. This makes sense: unpolarised light has oscillations distributed equally across all planes, and the filter passes only one plane's worth. The transmitted beam is now plane-polarised.

Place a second Polaroid filter after the first. If its transmission axis is parallel to the first, the already-polarised light passes through with little further reduction. Rotate the second filter by 90° so the axes are perpendicular, and the transmitted intensity falls to (ideally) zero — the crossed polarisers condition. This is a clean, repeatable experiment that demonstrates polarisation directly.

5. Applications of Polarisation

Polarisation is not a laboratory curiosity — it has real, widespread applications. The two the AQA specification requires you to know are Polaroid material (including sunglasses and photography filters) and the alignment of aerials for transmission and reception of television signals.

5a. Polaroid Sunglasses and Photography

When light reflects off a horizontal surface — water, a wet road, a car bonnet — the reflected beam becomes partially or fully polarised, with oscillations predominantly in the horizontal plane. Polaroid sunglasses have their transmission axis oriented vertically, so they block this horizontally polarised glare while still transmitting vertically polarised and unpolarised components of other light. The result is a significant reduction in glare without making everything appear dark.

Photographers use polarising filters on camera lenses for the same reason. Rotating the filter changes which plane of polarisation is transmitted, allowing the photographer to reduce reflections from glass or water, or to deepen the apparent blue of the sky (which is partially polarised due to scattering in the atmosphere).

5b. Aerial Alignment for TV Signals

Television signals are transmitted as electromagnetic waves — and electromagnetic waves are transverse. The transmitting aerial produces waves that are polarised in a specific plane, either horizontal or vertical depending on the broadcasting standard used in that region. In the UK, most terrestrial television transmitters broadcast horizontally polarised signals.

For a receiving aerial to pick up the maximum signal, it must be aligned in the same plane as the transmitted polarisation. The elements of a Yagi aerial (the horizontal rods you see on rooftops) must be oriented parallel to the plane of polarisation of the incoming signal. If the aerial is rotated 90° — so its elements are perpendicular to the polarisation plane — the received signal strength drops dramatically, approaching zero in the ideal case.

This is why all the TV aerials in a neighbourhood point in the same direction and have their elements in the same plane: they are all aligned to match the polarisation of the local transmitter. If you look at a rooftop aerial and the elements are horizontal, the signal is horizontally polarised. A misaligned aerial is the wave-physics equivalent of crossed polarisers — most of the signal is lost.

6. Comparing Polarisation in Different Contexts

It is worth pausing to compare the two main applications side by side, because exam questions sometimes ask you to draw connections between them. In both cases, the underlying physics is identical: a transverse wave has a plane of oscillation, and a receiver or filter aligned with that plane transmits or detects the wave efficiently, while one oriented perpendicularly does not.

7. Exam Technique and Mark-Winning Guidance

Polarisation questions appear in several forms at A Level: short 'state' or 'describe' questions, longer 'explain' questions worth 3–4 marks, and occasionally 'evaluate' questions asking you to assess polarisation as evidence for the transverse nature of waves. Each requires a different approach.

For a describe question about what happens when light passes through a Polaroid filter, focus on what changes: the oscillations are restricted to one plane, and the intensity is reduced. Do not explain why at this stage — the command word is 'describe', not 'explain'.

For an explain question about why polarisation is evidence for the transverse nature of light, you need a reasoning chain: (1) polarisation restricts oscillations to one plane; (2) this is only possible if oscillations exist in multiple perpendicular planes to begin with; (3) oscillations perpendicular to the direction of travel define a transverse wave; (4) therefore the ability to polarise light confirms it is transverse. Each step earns a mark — do not skip the logical links.

Taken together, the three specification requirements for this topic form a coherent argument: polarisation is a property unique to transverse waves (the physics), Polaroid material and aerial alignment are the two key applications you must be able to discuss (the context), and Malus's law is explicitly excluded so you do not need to calculate how intensity varies with angle (the boundary). Keep those three things clear and you have everything the examiner is looking for.