Waves and space · GCSE Physics
Transverse and longitudinal waves
Teacher-written GCSE Physics revision on transverse and longitudinal waves: oscillations versus energy transfer, amplitude, wavelength, frequency, period, and why sound needs a medium but light does not.
Waves transfer energy, not the medium itself. Transverse: oscillations at right angles to energy transfer. Longitudinal: oscillations parallel, as compressions and rarefactions.
The important bits
What you need to know
- 1
A wave transfers energy without transferring the material of the medium from end to end. A cork on water bobs; it does not ride to the shore with the wave.
- 2
Transverse waves: oscillations are perpendicular to the direction of energy transfer. Examples: water waves, waves on a string, all electromagnetic waves, S-waves.
- 3
Longitudinal waves: oscillations are parallel to the direction of energy transfer. Examples: sound, P-waves, a slinky pushed along its length. They show compressions and rarefactions.
- 4
Amplitude is the maximum displacement from the rest position, in metres. Larger amplitude means more energy, not a higher speed in the usual school examples.
- 5
Wavelength λ is the distance from one peak to the next, or from one compression to the next, in metres. Frequency f is the number of waves per second, in hertz (Hz).
- 6
Period T is the time for one wave, in seconds. T = 1/f, and f = 1/T. A frequency of 50 Hz has a period of 0.020 s.
- 7
Sound needs a medium: it is faster in solids than in liquids or gases because particles are closer. It cannot travel through a vacuum. Light can.
- 8
Seismic P-waves are longitudinal and travel through solids and liquids; S-waves are transverse and only through solids. That is how we know the outer core is liquid.
Quotations worth analysing
Short evidence. Real method.
“Waves transfer energy, not matter.”
The medium may oscillate, but it is not delivered to the receiver. Sound does not mean air from the speaker’s lungs arrives in your ear as a wind.
“In a transverse wave the oscillations are perpendicular to the direction of energy transfer.”
Draw a labelled diagram: arrows for oscillation up and down, a separate arrow for the wave along the tank. Two directions is the definition.
Go deeper
Draw two arrows or you have not defined the wave
Every classify question is a diagram in disguise. For a water wave, one arrow bobs the surface up and down, the other points towards the beach: they are at right angles, so the wave is transverse. For sound, particles of air oscillate along the same line the energy travels, crowding into compressions and spreading into rarefactions: longitudinal. A slinky can do both, which is why teachers use it. Frequency is how often a point oscillates; wavelength is how far it is to the next point doing the same thing. Students mix amplitude with wavelength (“taller waves are longer”) or say sound is transverse because they have seen a sine curve on an oscilloscope. The oscilloscope is a graph of displacement against time, not a photograph of a transverse ripple. Name the oscillations in the medium.
Go deeper
Medium, vacuum and the Earth are the application marks
Sound is a mechanical longitudinal wave: no particles, no sound. That is why space is silent and why a bell in a vacuum jar fades as the air is pumped out. Light is a transverse electromagnetic wave and does not need a medium, which is why we see the Sun. Earthquakes send both types. P-waves arrive first and travel through the molten outer core; S-waves do not, leaving a shadow zone. You do not need a geophysics A-level, but you should be able to say transverse waves cannot propagate in a liquid in that context. Amplitude on a seismometer or a loudspeaker cone is still maximum displacement. Frequency is still 1/T. The definitions do not change just because the example got larger.
See the idea in action
A tuning fork produces sound at 256 Hz. Period T = 1/f = 1/256 = 0.00391 s, about 3.9 ms. If the wavelength in air is 1.33 m, the wave is longitudinal: compressions 1.33 m apart. A water ripple with the same frequency would be transverse, with crests 1.33 m apart only if its speed happened to match — in water it would not. The cork’s amplitude might be 4.0 mm = 0.0040 m; that is not λ. If 20 crests pass a post in 5.0 s, f = 20 / 5.0 = 4.0 Hz, T = 0.25 s. Count the gaps between crests for wavelength, not the number of crests piled on the page.
Exam technique
Turn knowledge into marks
State transverse or longitudinal with the two directions: oscillations versus energy transfer. Give a named example of each. Sound is longitudinal and needs a medium; light is transverse and does not. T = 1/f with T in seconds and f in hertz.
Common mistakes
Do not give these marks away
- 01
Saying waves transfer particles, or that sound travels through a vacuum.
- 02
Calling sound transverse because of a sine-wave sketch on an oscilloscope.
- 03
Mixing up amplitude and wavelength, or using T = f instead of T = 1/f.
Which statement is correct?
ASound is a transverse wave that travels through a vacuum
BLight is a longitudinal wave that needs air
CSound is a longitudinal wave; oscillations are parallel to energy transfer and a medium is required
DWater waves are longitudinal because the water moves to the shore
Show the answer
Sound is a longitudinal wave; oscillations are parallel to energy transfer and a medium is required. Sound is compressions and rarefactions in a medium. Light is transverse and can cross a vacuum. Water waves are transverse; the water mainly oscillates rather than travelling with the wave.
Quick questions
If this is the bit you searched
What is the difference between a transverse and a longitudinal wave?
Transverse: oscillations at right angles to energy transfer (light, water). Longitudinal: oscillations parallel to energy transfer (sound), as compressions and rarefactions.
What is amplitude?
The maximum displacement of a point on the wave from its rest position, measured in metres. It is linked to the energy of the wave, not to wavelength.
Why can light travel through space but sound cannot?
Light is electromagnetic and does not need a medium. Sound is a mechanical wave: it needs particles to compress and rarefy.
How are frequency and period linked?
T = 1/f. Frequency in hertz is waves per second; period is the number of seconds per wave.