Properties of Waves Notes

Edexcel GCSE Physics: Revision notes

Key facts

  • Waves transfer energy and information, not matter.
  • Transverse waves vibrate at right angles to the direction of energy transfer; longitudinal waves vibrate parallel to it.
  • Wave speed v=fλv=f\lambda (frequency × wavelength) or v=xtv=\dfrac{x}{t}.
  • Frequency is in hertz (Hz), wavelength in metres, amplitude from the rest line to a crest.
  • At a boundary a wave can be reflected, refracted, transmitted or absorbed. Refraction is caused by a change in speed.

What waves transfer

Waves carry energy and information from place to place, but the material they pass through does not travel along.

The substance a wave travels through is the medium.

  • A cork on water bobs up and down as a ripple passes, but does not move along with it.
  • A sound wave makes air particles vibrate back and forth. They pass the disturbance on to their neighbours and stay in about the same place.
  1. 1

    Source vibrates

    it disturbs the nearest part of the medium

  2. 2

    Particles pass it on

    each vibrates about a fixed position and disturbs its neighbour

  3. 3

    Energy arrives

    the receiver gains energy; the medium has not travelled

How a wave moves energy without moving the medium.

Transferred

  • Energy
  • Information

Not transferred

  • Matter: the medium stays in place

A ripple passes a floating cork. What does the cork do?

Wave quantities

Frequency, wavelength, amplitude and period describe a wave, and each has its own unit.

Frequency: complete waves per second, in hertz (Hz).

Wavelength: crest to crest, in metres.

Amplitude: maximum displacement from the rest position.

Period: time for one wave, in seconds.

Wavefront: a line joining points in phase, such as all at a crest.

12345678−3−2−1123xycrestnext cresttroughdisplacement
Displacement against distance: amplitude is measured from the rest line to a crest; wavelength is crest to crest (here 4). Drag a.
  • PeriodT=1fT=\dfrac{1}{f}

Which quantity is measured from crest to crest?

Transverse and longitudinal

The direction of vibration compared with the direction of energy transfer decides the type.

In a longitudinal wave the particles bunch up (compressions) and spread out (rarefactions).

Seismic waves come in both types: S-waves are transverse and P-waves are longitudinal.

24681012−1.5−1−0.50.511.5xycompressionrarefactionPressure variation
Pressure in a sound (longitudinal) wave: the peaks are compressions and the troughs are rarefactions.

Transverse

Vibration direction:
At right angles to the energy transfer
Examples:
Electromagnetic, water and seismic S-waves

Longitudinal

Vibration direction:
Parallel to the energy transfer (compressions and rarefactions)
Examples:
Sound and seismic P-waves

Which of these is a longitudinal wave?

Wave speed

Wave speed is frequency times wavelength, or distance divided by time.

To measure the speed of sound in air, two people stand a known long distance apart. One makes a loud sound; the other times from seeing it to hearing it. Repeat and average.

For ripples, time one across a measured distance in a ripple tank.

(Higher Tier) For an echo, x=v×tx=v\times t for the journey down and back, so halve it for the one-way distance.

10020030040050060070080090010001234567xy170 Hz, 2 m340 Hz, 1 m680 Hz, 0.5 mλ = 340 ÷ f
For sound in air (v = 340 m/s), wavelength falls as frequency rises: λ = 340 ÷ f.
  • Wave speedv=fλv=f\lambda
  • Wave speedv=xtv=\dfrac{x}{t}

Worked example

A wave has frequency 50 Hz and wavelength 2 m. Calculate its speed.

A wave has frequency 10 Hz and wavelength 3 m. What is its speed?

Waves at boundaries

A wave can be reflected, refracted, transmitted or absorbed, depending on the materials.

Refraction happens because the wave changes speed crossing the boundary. If it meets the boundary at an angle, the change in speed also changes its direction.

(Higher Tier) Different substances reflect, absorb, transmit or refract waves differently depending on wavelength, so some materials block certain colours but let others through.

  1. 1

    Meets at an angle

    Not along the normal

  2. 2

    Speed changes

    The new material changes the speed

  3. 3

    Direction changes

    The wave is refracted

Why a wave changes direction at a boundary

Reflected

  • Bounces back

Refracted

  • Changes direction as its speed changes

Transmitted

  • Passes through

Absorbed

  • Energy taken in, often to the thermal store

What causes refraction?

Try an exam question

A wave has a frequency of 50 Hz and a wavelength of 2 m. (a) Calculate the speed of the wave. (b) State the difference between a transverse wave and a longitudinal wave.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.