Properties of Waves Notes

AQA GCSE Physics: Revision notes

Key facts

  • Waves transfer energy, not matter.
  • Transverse waves oscillate at right angles to the energy transfer (light, ripples); longitudinal waves oscillate parallel to it (sound).
  • Amplitude is the maximum displacement; wavelength is the distance between equivalent points on successive waves.
  • T=1fT = \dfrac{1}{f} and wave speed v=fλv = f\lambda.

Transverse and longitudinal

The two types differ in how the oscillations line up with the direction the energy travels.

In transverse waves the oscillations are at right angles to the energy transfer: water ripples, light and all other electromagnetic waves.

In longitudinal waves they are parallel to it: sound waves show compressions (particles squeezed together) and rarefactions (particles spread apart).

A slinky pushed along its length models a longitudinal wave; shaken side to side it models a transverse wave.

Transverse

Oscillation:
Perpendicular to energy transfer
Pattern:
Peaks and troughs
Examples:
Light, water ripples

Longitudinal

Oscillation:
Parallel to energy transfer
Pattern:
Compressions and rarefactions
Examples:
Sound

Which wave is longitudinal?

Energy moves, matter does not

No: the wave carries energy from place to place while the particles only oscillate about a fixed position.

This is true for ripples on water and for sound in air. The water or air particles do not travel with the wave.

In a Mexican wave at a stadium each person stands and sits in place, but the wave travels around the stadium.

The wave

Does it travel along?:
Yes
What it carries:
Energy

The particles of the medium

Does it travel along?:
No: they oscillate about a fixed position
What it carries:
Nothing along with the wave

A cork floats on a water ripple. What happens to it as the ripple passes?

Describing a wave

Every wave has an amplitude, wavelength, frequency and period.

  • Amplitude: the maximum displacement of a point on the wave from its undisturbed position.
  • Wavelength (λ\lambda): the distance from a point on one wave to the equivalent point on the next.
  • Frequency (ff): the number of complete waves passing a point per second, in hertz (Hz).
  • Period (TT): the time for one complete wave to pass a point.
12345678−3−2−1123xycrest (1, 2)trough (3, -2)one wavelength = 4displacement
Transverse wave: amplitude 2, wavelength 4. Slide to change amplitude.

What does the amplitude of a wave measure?

Period, frequency and speed

Period is the reciprocal of frequency, and wave speed is frequency multiplied by wavelength.

Period is in seconds, frequency in hertz, wave speed in m/s and wavelength in metres.

Amplitude and wavelength can be read from a wave trace. The speed of sound in air and of ripples on water can be measured by timing how long they take to travel a known distance.

  • Period (s)T=1fT = \dfrac{1}{f}
  • Wave speed (m/s)v=fλv = f\lambda

Worked example

A wave has a frequency of 50 Hz and a wavelength of 2 m. Find its speed and period.

A wave has a frequency of 4 Hz. What is its period?

Try an exam question

A wave on a rope has a frequency of 5 Hz and a wavelength of 0.6 m. Write down the equation you use and calculate the wave speed.

[3 marks]

That's the notes covered.

Carry on to the next subtopic.