Properties of WavesAQA GCSE Physics: Revision notes
Section 1
What Is the Difference Between Transverse and Longitudinal Waves?
Waves transfer energy without transferring matter. There are two types, defined by the direction of oscillation relative to the direction of energy transfer:
- Transverse waves — oscillations are at right angles (perpendicular) to the direction of energy transfer, e.g. water ripples, light and all other electromagnetic waves.
- Longitudinal waves — oscillations are parallel to the direction of energy transfer, e.g. sound waves, which show compressions (particles squeezed together) and rarefactions (particles spread apart).
A slinky spring pushed and pulled along its length models a longitudinal (sound) wave; a slinky shaken side to side models a transverse wave.
Section 2
Does the Medium Itself Travel With the Wave?
No. For both ripples on water and sound waves in air, it is the wave that travels — carrying energy from one place to another — while the water or air particles themselves only oscillate around a fixed position and do not travel with the wave.
A wave moving through a crowd doing a 'Mexican wave' at a stadium — each person just stands up and sits down in place, but the wave itself travels around the stadium.
Section 3
How Do We Describe a Wave?
Every wave can be described using these quantities:
- Amplitude — the maximum displacement of a point on the wave from its undisturbed position.
- Wavelength (λ) — the distance from a point on one wave to the equivalent point on the next wave.
- Frequency (f) — the number of complete waves passing a point per second, measured in hertz (Hz).
- Period (T) — the time taken for one complete wave to pass a point.
Section 4
How Do You Calculate Period, Frequency and Wave Speed?
Two key equations link these wave quantities:
- Period and frequency: T = 1/f (period in seconds, frequency in hertz)
- Wave equation: v = f × λ (wave speed in m/s, frequency in Hz, wavelength in m)
Amplitude and wavelength can be identified by reading measurements directly from a wave trace or graph. The speed of sound in air, and the speed of ripples on a water surface, can both be measured experimentally, for example by timing how long a sound or ripple takes to travel a known distance.
A wave has a frequency of 50 Hz and a wavelength of 2 m. Its speed = f × λ = 50 × 2 = 100 m/s.
Do not confuse frequency (waves per second) with period (seconds per wave) — they are reciprocals of each other, T = 1/f.
Must Know
- Transverse waves oscillate perpendicular to energy transfer (e.g. light, water ripples); longitudinal waves oscillate parallel to it (e.g. sound), showing compressions and rarefactions.
- Waves transfer energy, not matter — the medium's particles oscillate but do not travel with the wave.
- Amplitude = maximum displacement from undisturbed position; wavelength = distance between equivalent points on successive waves.
- Frequency = number of waves per second (Hz); period = time for one wave to pass (T = 1/f).
- Wave speed = frequency × wavelength (v = fλ).
- The speed of sound in air and of water ripples can be measured experimentally.
That's the notes covered.
Carry on to the next subtopic.