Longitudinal vs. Transverse Waves Revision Notes | A Level AQA Physics

1. The Core Question: Which Way Do Particles Move?

Every wave transfers energy from one place to another. The critical question — the one that separates longitudinal from transverse — is this: does the medium move with the wave, or across it? That single distinction underpins everything in this topic.

Picture a crowd doing a Mexican wave in a stadium. Each person stands up and sits down — they move vertically — yet the wave itself travels horizontally around the ground. The displacement of each person is perpendicular to the direction the wave travels. That is a transverse wave in everyday life.

Now picture pushing a slinky spring along a table. You compress one end and the compression travels along the spring in the same direction as your push. The coils move parallel to the direction of energy travel. That is a longitudinal wave. Hold both images in mind — they are your anchors for everything that follows.

2. Transverse Waves — Displacement Perpendicular to Propagation

Consider a wave on a string. Fix one end of a rope to a wall and shake the other end up and down. Energy travels horizontally towards the wall, but every point on the rope moves vertically — up, back to rest, down, back to rest. The displacement of each point is at 90° to the direction of energy propagation. That perpendicular relationship is the defining feature of a transverse wave.

Electromagnetic waves follow the same rule, but with a twist: there is no physical medium. Instead, oscillating electric and magnetic fields carry the energy. Both fields oscillate perpendicular to the direction of travel — and perpendicular to each other. This is why light can travel through the vacuum of space, where there are no particles to displace.

The chart above shows a snapshot of a transverse wave. The horizontal axis is position along the string — the direction energy travels. The vertical axis is the displacement of each point on the string. Notice that the displacement is perpendicular to the direction of propagation: this is the visual signature of a transverse wave.

3. Longitudinal Waves — Displacement Parallel to Propagation

Now shift attention to the other category. In a longitudinal wave, particles oscillate along the same line as the direction of energy travel. This creates alternating regions of high pressure (compressions) and low pressure (rarefactions) as particles bunch together and then spread apart.

Sound is the go-to example. When a loudspeaker cone moves forward, it pushes air molecules together — a compression. When it moves back, it leaves a region where molecules are spread out — a rarefaction. These pressure variations travel outward from the speaker at approximately 340 m/s in air at room temperature. Your eardrum detects the pressure changes, not the individual air molecules arriving from the speaker.

A key consequence of this parallel oscillation is that longitudinal waves cannot be polarised. Polarisation restricts oscillation to a single plane, which is only possible when oscillation is perpendicular to propagation. The fact that sound cannot be polarised is itself evidence that it is longitudinal — a point examiners occasionally probe.

4. Comparing Longitudinal and Transverse Waves

Having established both types separately, it is worth placing them side by side. AQA examiners frequently ask you to compare — meaning you must address both similarities and differences explicitly. The table below gives you the framework.

Notice that both wave types transfer energy without transferring matter — this is a shared property of all progressive waves. The distinction is purely about the geometry of the oscillation relative to the propagation direction.

5. Electromagnetic Waves — A Special Case of Transverse Waves

Electromagnetic waves deserve their own section because they have a property that sets them apart from all mechanical waves: they travel through a vacuum. There are no particles to oscillate — instead, it is oscillating electric and magnetic fields that constitute the wave. Both fields are perpendicular to the direction of travel and perpendicular to each other, confirming the transverse nature.

The AQA specification requires you to know that all electromagnetic waves travel at the same speed in a vacuum. That speed is $c = 3.0 \times 10^8$ m/s. This applies to every member of the electromagnetic spectrum — radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays — regardless of their frequency or wavelength.

Although all EM waves share the same speed in a vacuum, they differ in frequency and wavelength. The wave equation $v = f\lambda$ still applies: a higher frequency means a shorter wavelength, and vice versa, because the speed is fixed. This is why gamma rays have tiny wavelengths and radio waves have enormous ones — their frequencies differ by many orders of magnitude.

When EM waves enter a medium such as glass or water, they slow down. The speed in a vacuum, $c$, is the maximum — and the value you use unless a question specifies a different medium. In air, the speed is so close to $c$ that for most AQA calculations you treat them as equal.

6. Worked Example — Applying the Wave Equation to an EM Wave

The wave equation connects speed, frequency, and wavelength. For EM waves in a vacuum, the speed is always $c = 3.0 \times 10^8$ m/s. Exam questions often give you one of frequency or wavelength and ask you to find the other. Here is a model walkthrough.

Worked example: Calculating the Wavelength of a Radio Wave

A VHF radio transmitter broadcasts at a frequency of $1.5 \times 10^8$ Hz. Calculate the wavelength of the radio waves in a vacuum. [3 marks]

1. $v = f\lambda$ — [M1] Selecting the correct equation. For EM waves in a vacuum, $v = c = 3.0 \times 10^8$ m/s.
2. $\lambda = \frac{v}{f}$ — Divide both sides by $f$ to isolate $\lambda$. This is the rearrangement step — show it explicitly for method marks.
3. $\lambda = \frac{3.0 \times 10^8}{1.5 \times 10^8}$ — [M1] Substituting $v = 3.0 \times 10^8$ m/s and $f = 1.5 \times 10^8$ Hz with correct units.
4. $\lambda = \frac{3.0}{1.5} \times \frac{10^8}{10^8} = 2.0 \times 10^0 = 2.0$ m — $3.0 \div 1.5 = 2.0$ and $10^8 \div 10^8 = 10^0 = 1$. Carry out numerics and powers separately to avoid errors.
5. $\lambda = 2.0$ m — [A1] The wavelength is 2.0 m. Always include units — omitting them loses the accuracy mark. A 2 m wavelength is characteristic of the VHF (Very High Frequency) band, which spans roughly 1–10 m; note that typical FM broadcast radio operates at around 100 MHz (~3 m), so this example uses a slightly higher VHF frequency chosen to give a clean calculation.

Answer: $\lambda = 2.0$ m

Notice how the powers of ten were handled separately from the numerical coefficients. This is the safest approach with standard form: divide the coefficients first ($3.0 \div 1.5 = 2.0$), then deal with the powers ($10^8 \div 10^8 = 1$). Combining them in one rushed step is where arithmetic errors creep in.

7. Exam Technique — Scoring Full Marks on Wave Questions

Wave questions at A Level are rarely just recall. Examiners use command words like describe, explain, and compare to probe whether you understand the geometry, not just the label. Here is how to translate your understanding into marks.

When a question says describe a transverse wave, give the direction relationship explicitly: state that particle (or field) displacement is perpendicular to the direction of energy propagation. Naming an example alone is not sufficient for a describe question — you must characterise the motion.

When a question says compare longitudinal and transverse waves, you must address both types and use comparative language. A response that only describes one type will not access the higher marks. Structure your answer as: similarity — both transfer energy without net transfer of matter; difference — displacement direction relative to propagation.

Across this lesson, every spec requirement maps to a clear geometric principle: transverse means perpendicular displacement, longitudinal means parallel displacement, and all electromagnetic waves — as transverse waves — share a vacuum speed of $3.0 \times 10^8$ m/s. Anchor your exam answers to these relationships and you will access the marks that vague descriptions miss.