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This lesson introduces the two main types of wave — transverse and longitudinal — and the key properties used to describe them, as required by the Edexcel GCSE Combined Science specification (1SC0). A secure understanding of wave terminology is essential for every topic that follows in this unit.
A wave is a disturbance that transfers energy from one place to another without transferring matter. The particles of the medium vibrate about a fixed (equilibrium) position but do not travel with the wave.
Exam Tip: A very common exam question asks what waves transfer. The answer is always energy, never matter or particles.
In a transverse wave the oscillations (vibrations) of the particles are perpendicular (at right angles) to the direction of energy transfer.
| Feature | Detail |
|---|---|
| Oscillation direction | Perpendicular to energy transfer |
| Can travel through | Solids, liquids and vacuums |
| Examples | Water surface waves, light and all EM waves, S-waves (seismic), waves on a string |
graph LR
subgraph "Transverse Wave"
direction LR
A["Energy transfer →"] --- B["↑ Crest"]
B --- C["↓ Trough"]
C --- D["↑ Crest"]
D --- E["↓ Trough"]
end
The peaks are called crests and the lowest points are called troughs.
In a longitudinal wave the oscillations of the particles are parallel to the direction of energy transfer.
| Feature | Detail |
|---|---|
| Oscillation direction | Parallel to energy transfer |
| Can travel through | Solids, liquids and gases |
| Examples | Sound waves, ultrasound, P-waves (seismic) |
graph LR
subgraph "Longitudinal Wave"
direction LR
A["|||| Compression"] --- B[" | | | Rarefaction"]
B --- C["|||| Compression"]
C --- D[" | | | Rarefaction"]
end
Exam Tip: Remember the mnemonic: Longitudinal — Like a sLinky pushed and pulled along its length; Transverse — The vibrations go Top-to-bottom (perpendicular).
All waves — whether transverse or longitudinal — can be described using the same set of properties.
| Property | Symbol | Unit | Definition |
|---|---|---|---|
| Amplitude | A | metres (m) | Maximum displacement of a point on the wave from its equilibrium (rest) position |
| Wavelength | λ (lambda) | metres (m) | Distance between two consecutive points in phase (e.g. crest to crest) |
| Frequency | f | hertz (Hz) | Number of complete waves passing a point per second |
| Period | T | seconds (s) | Time taken for one complete wave to pass a point |
The relationship between frequency and period is:
$$f = \frac{1}{T}$$
and equivalently:
$$T = \frac{1}{f}$$
| Quantity | Symbol | Unit |
|---|---|---|
| Frequency | f | Hz (s⁻¹) |
| Period | T | s |
A wave has a period of 0.02 s. Calculate its frequency.
$$f = \frac{1}{T} = \frac{1}{0.02} = 50 \text{ Hz}$$
A radio station broadcasts at a frequency of 200 000 Hz. What is the period of the wave?
$$T = \frac{1}{f} = \frac{1}{200,000} = 5 \times 10^{-6} \text{ s}$$
Exam Tip: Make sure you can rearrange f = 1/T for both f and T. These quick calculations often appear as 1-mark questions.
| Feature | Transverse | Longitudinal |
|---|---|---|
| Oscillation direction | Perpendicular to energy transfer | Parallel to energy transfer |
| Can be polarised? | Yes | No |
| Can travel through a vacuum? | Only EM waves | No |
| Examples | Light, water waves, S-waves | Sound, ultrasound, P-waves |
Polarisation is the restriction of a transverse wave's oscillations to a single plane. Only transverse waves can be polarised. This is evidence that light is a transverse wave.
When you are given a displacement–distance graph (a snapshot of a transverse wave):
When you are given a displacement–time graph (how one point moves over time):
A displacement–distance graph shows a wave with crests at 0 m and 0.6 m along the x-axis, and an amplitude of 0.04 m.
If the wave has a frequency of 5 Hz, calculate its period.
$$T = \frac{1}{f} = \frac{1}{5} = 0.2 \text{ s}$$
| Misconception | Correction |
|---|---|
| Waves move matter from place to place | Waves transfer energy, not matter |
| Amplitude is the distance from crest to trough | Amplitude is the distance from the rest position to the crest (half the crest-to-trough distance) |
| Longitudinal waves don't have a wavelength | They do — measured from compression to compression |
| Sound is a transverse wave | Sound is a longitudinal wave |