General Properties of Waves Notes

Oxford AQA IGCSE Physics: Revision notes

Key facts

  • A wave transfers energy and information, not matter.
  • Transverse waves oscillate perpendicular to the energy transfer; longitudinal waves oscillate parallel to it.
  • At a boundary a wave can be reflected, transmitted or absorbed.
  • Know frequency, wavelength, period, amplitude and wavefront.
  • Wave speed v=f×λv = f \times \lambda for all waves.

What a wave is

A wave is a disturbance from an oscillating source that carries energy and information from place to place without moving matter.

Particles of the medium oscillate about a fixed position but do not travel with the wave. Only energy and information move.

A cork on a pond bobs up and down as a wave passes, then stays where it was.

  1. 1

    Source oscillates

    for example a vibrating object in the water

  2. 2

    Particles oscillate

    each particle moves about a fixed position and passes the energy on

  3. 3

    Energy travels, matter does not

    the cork bobs up and down, then stays where it was

A wave moving across a pond.
  • Wavetransfers energy and information, not matter

A cork floats on a pond and a wave passes. What happens to the cork?

Transverse and longitudinal

The difference is the direction of oscillation compared with the direction the energy travels.

In a transverse wave the oscillations are perpendicular to the direction of energy transfer. In a longitudinal wave they are parallel to it.

Electromagnetic waves and water waves are transverse. Sound waves are longitudinal.

24681012−3−2−1123xydisplacement
A transverse wave: peaks and troughs, with oscillation perpendicular to the direction of travel (illustrative values).

Transverse

Oscillation:
Perpendicular to energy transfer
Examples:
Electromagnetic and water waves
Pattern:
Peaks and troughs

Longitudinal

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

In which type of wave do particles oscillate parallel to the energy transfer?

Waves at a boundary

A wave meeting a boundary can be reflected, transmitted or absorbed, and it may refract or diffract.

At a boundary a wave can be reflected (bounced back), transmitted (passed into the new material) or absorbed (energy taken in).

Refraction is the change in speed, and usually direction, as a wave crosses a boundary. Diffraction is the spreading of a wave through a narrow gap or past an edge.

  1. 1

    Reflected

    Bounced back from the boundary

  2. 2

    Transmitted

    Passes into the new material, often refracting

  3. 3

    Absorbed

    Wave energy is taken in

Three things that can happen at a boundary

What is diffraction?

Describing waves

Five quantities describe a wave: frequency, wavelength, period, amplitude and wavefront.

  • Frequency: complete waves passing a point per second, in hertz (Hz).
  • Wavelength: distance between identical points on neighbouring waves.
  • Period: time for one complete wave to pass a point.
  • Amplitude: maximum displacement from the rest position.
  • Wavefront: a line joining points in phase.
24681012−4−3−2−11234xyy = a sin(2πx/λ)
A transverse wave: change the amplitude and wavelength with the sliders.

Which property is the maximum displacement from the rest position?

Wave speed

Wave speed is frequency multiplied by wavelength, and this applies to every type of wave.

Use speed vv in m/s, frequency ff in Hz and wavelength λ\lambda in m. Rearrange to find any one of them.

12345678910246810xyλ = v / f
For a fixed wave speed v, wavelength falls as frequency rises: λ = v / f. Drag v (illustrative values).
  • Wave speedv=f×λv = f \times \lambda

Worked example

A wave has a frequency of 50 Hz and a wavelength of 2 m. What is its speed?

A wave travels at 340 m/s with a frequency of 170 Hz. What is its wavelength?

Try an exam question

A wave has a frequency of 20 Hz and a wavelength of 3 m. Calculate its speed, and state what the wave transfers.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.