What a wave is
A wave transfers energy from one place to another without transferring matter. The particles (or fields) vibrate about fixed positions; they do not travel with the wave.
Two types of wave
- Transverse: the vibrations are at right angles to the direction the wave travels. Examples: water waves, light and all electromagnetic waves.
- Longitudinal: the vibrations are along the direction of travel, producing compressions (particles close together) and rarefactions (particles spread out). Example: sound.
A line joining points on a wave that are all in step, such as the tops of the crests, is called a wavefront. Neighbouring wavefronts are one wavelength apart, and the wave travels at right angles to them.
Describing waves
Key quantities are: amplitude (maximum displacement from rest, linked to energy), wavelength λ (distance between two neighbouring points in step, e.g. crest to crest), frequency f (number of complete waves passing a point each second, in hertz, Hz) and period T (time for one complete wave to pass). The frequency is set by the source producing the wave and does not change when the wave passes into a new material.
Key idea
Wave speed: v = f λ
Period and frequency: T = 1 / f
(v in m/s, f in Hz, λ in m, T in s)
Worked example
A wave has a frequency of 50 Hz and a wavelength of 4.0 m. Find its speed.
v = f λ = 50 × 4.0 = 200 m/s. Its period T = 1/f = 1/50 = 0.020 s.
Remember
- A wave carries energy, not matter.
- A higher frequency means a shorter wavelength (for the same speed).
- Larger amplitude means more energy is carried.