Physics 4.1 - 4.5 - Wave behaviour and properties
Follow a travelling wave while its particles oscillate locally. Use amplitude, wavelength, frequency, period and wavefronts to describe the pattern, and compare transverse and longitudinal waves.
What travels in a wave?
A wave is a travelling disturbance. A source begins an oscillation, and interactions
between neighbouring parts of a medium can pass that disturbance onwards. The pattern can
travel a long distance even though each piece of the medium moves only around its own
equilibrium, or rest, position.
Wave transfer
Waves transfer energy and information from place to place without an overall
transfer of matter.
"Without transferring matter" does not mean that matter never moves. In a mechanical
wave, particles of the medium must oscillate locally. It means that the particles are not
carried all the way from the source to the receiver with the wave.
The distinction is easiest to see by tracking one marked part of the medium:
- Water-wave evidence: place a small floating marker on still water and send ripples
across it. The wavefront travels across the surface, but the marker mainly bobs around
the same horizontal position. The water is disturbed locally rather than transported
across the tank with a crest. - Sound-wave evidence: a loudspeaker cone moves backwards and forwards. It creates
moving regions in which the air is alternately compressed and spread out, while each
small region of air moves to and fro about its equilibrium position. A microphone
diaphragm farther away also vibrates to and fro; the block of air next to the speaker
does not travel all the way to the microphone.
The travelling disturbance carries energy because it can cause a change at the
receiver: a water ripple can make the floating marker move, and a sound wave can make an
eardrum or microphone diaphragm vibrate. It can carry information when variations in
the disturbance form a signal. The changing pattern in a sound wave, for example, can
represent speech or music.
This is a causal chain:
oscillating source → neighbouring regions respond → disturbance travels → receiver oscillates
The source and receiver do not exchange the same piece of matter. Energy and a patterned
signal pass through the chain.
Describing a repeating wave
A wave model needs two directions. The direction of travel is the direction in which
the disturbance and energy propagate. The direction of oscillation is the direction in
which a point in the medium, or the changing disturbance, moves about its equilibrium.
Keeping those directions separate prevents a wave sketch from being mistaken for the path
of a particle.
Six terms describe a repeating wave:
| Term | Meaning | Unit or representation |
|---|---|---|
| amplitude, A | maximum displacement from the equilibrium position | metre, |
| wavelength, lambda | shortest distance between two successive points at the same stage of the cycle, measured parallel to wave travel | metre, |
| frequency, f | number of complete waves or cycles passing a fixed point each second | hertz, ; means one cycle per second |
| period, T | time taken for one complete cycle at a point | second, |
| wave velocity, v | speed and direction of the travelling disturbance or wavefront; speed calculations use its magnitude | metre per second, |
| wavefront | a line, or surface in three dimensions, joining points at the same stage of a wave cycle | for water ripples, one crest line is a wavefront |
[DIAGRAM: asset_slug: phy_c_wave_properties_verified; description: Crest-to-crest wavelength; equilibrium-to-crest amplitude; adjacent straight wavefronts with travel perpendicular to them.]
Amplitude is measured from the equilibrium line to a crest or to a trough. Crest-to-trough
height is , so measuring the full height and calling it the amplitude gives a factor-of-two
error.
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