What is a wave?
A wave is a travelling disturbance that transfers energy from one point to another without the transfer of matter. The particles of the medium only vibrate (oscillate) about fixed positions; they do not travel along with the wave.
- Transverse wave: particles vibrate perpendicular to the direction of wave travel (e.g. water surface waves, light).
- Longitudinal wave: particles vibrate parallel to the direction of wave travel, forming compressions and rarefactions (e.g. sound).
Describing a wave
Key quantities describe every wave:
- Amplitude (a): maximum displacement from the rest position; linked to the energy carried.
- Wavelength (λ): distance between two consecutive points in phase (e.g. crest to crest), in metres.
- Period (T): time for one complete oscillation, in seconds.
- Frequency (f): number of complete oscillations per second, in hertz (Hz).
Key formula / Key formula
v = fλ (wave speed = frequency × wavelength)
T = 1/f and f = 1/T
The wave equation in use
Because speed, frequency and wavelength are linked by v = fλ, if any two are known the third can be found. Frequency is fixed by the source; when a wave enters a new medium its frequency stays the same.
Worked example
A wave on a rope has a frequency of 5 Hz and a wavelength of 0.40 m. Find its speed and period.
Speed: v = fλ = 5 × 0.40 = 2.0 m s⁻¹.
Period: T = 1/f = 1/5 = 0.20 s.
Remember
- A wave moves energy, not matter.
- Frequency is set by the source and does not change between media.
- Use consistent SI units: metres, seconds, hertz.