Properties of WavesEdexcel GCSE Physics: Revision notes
Section 1
What do waves actually transfer?
Waves transfer energy and information from one place to another, without transferring matter — the substance the wave travels through (the medium) does not move along with the wave.
- A cork floating on water bobs up and down as a ripple passes, but does not travel with the wave — this shows it is the wave, not the water, that moves outward
- Sound waves travel through air by making air particles vibrate back and forth; the particles themselves stay in roughly the same average position, only passing the disturbance on to neighbouring particles
Think of a Mexican wave in a stadium: the 'wave' travels around the ground, but each person only stands up and sits down — they don't move around the stadium with it.
Section 2
What are the key wave quantities?
- Frequency (f): the number of complete waves passing a point per second, measured in hertz (Hz)
- Wavelength (λ): the distance from one point on a wave to the identical point on the next wave (e.g. crest to crest), measured in metres
- Amplitude: the maximum displacement of a point on the wave from its undisturbed (rest) position
- Period (T): the time taken for one complete wave to pass a point, in seconds
- Wave velocity: the speed at which the wave energy travels, in metres per second
- Wavefront: a line joining points on a wave that are in phase (e.g. all at a crest), showing the wave's shape and direction of travel
Section 3
What is the difference between longitudinal and transverse waves?
| Type | Vibration direction | Examples |
|---|---|---|
| Transverse | At right angles to the direction of energy transfer | Electromagnetic waves, water waves, seismic S-waves |
| Longitudinal | Parallel to (along) the direction of energy transfer, forming compressions and rarefactions | Sound waves, seismic P-waves |
In a longitudinal wave, particles bunch together (compressions) and spread apart (rarefactions) as the wave passes.
Students often forget seismic waves come in both types — S-waves are transverse, P-waves are longitudinal.
Section 4
How do we calculate wave speed?
Two key equations:
- v = f × λ (wave speed = frequency × wavelength)
- v = x / t (wave speed = distance ÷ time)
To measure the speed of sound in air: two people stand a known large distance apart; one bangs a drum or claps, the other starts a stopwatch on seeing the flash/action and stops it on hearing the sound; speed = distance ÷ time (repeated and averaged to reduce error).
To measure the speed of ripples on a water surface: time how long a ripple takes to travel a measured distance across a ripple tank, or count how many wavelengths pass a fixed point per second (frequency) and measure the wavelength directly.
Higher tier: v = x/t can be rearranged to find depth or distance when a wave (e.g. an echo) reflects back — for example, x = v × t for a wave travelling down and back.
A wave has frequency 50 Hz and wavelength 2 m. Speed = f × λ = 50 × 2 = 100 m/s.
Always state the equation and substitute values with units shown — mark schemes give a mark just for writing v = fλ before calculating.
Section 5
What happens when a wave meets a boundary between materials?
At an interface between two materials, a wave can be:
- Reflected — bounced back off the surface
- Refracted — passes into the new material but changes direction, because its speed changes
- Transmitted — passes through the material
- Absorbed — energy is taken in by the material (often converted to heat)
Refraction happens because a wave changes speed when it crosses a boundary into a material where it travels at a different speed; if it enters at an angle (not along the normal), this speed change also causes it to change direction.
Higher tier: different substances absorb, transmit, refract or reflect waves differently depending on the wave's wavelength — this is why, for example, some materials block certain colours of light but let others through.
Must Know
- Waves transfer energy and information, not matter — the medium itself does not travel with the wave
- v = f × λ and v = x/t are the two key wave speed equations
- Transverse waves vibrate at right angles to travel direction (light, water, S-waves); longitudinal waves vibrate parallel to it, with compressions and rarefactions (sound, P-waves)
- Amplitude, wavelength, frequency, period and wavefront are all measurable wave properties
- At a boundary, waves can be reflected, refracted, transmitted or absorbed
- Refraction is caused by a change in wave speed when crossing into a different material
That's the notes covered.
Carry on to the next subtopic.