IGCSEPhysicsOxford AQARevision NotesWavesGeneral Properties of WavesWave Speed

Wave Speed (Oxford AQA IGCSE Physics)

Revision Note

Leander Oates

Expertise

Physics

Wave Speed

  • Wave speed is defined as:

The distance travelled by a wave each second

  • The wave speed can be calculated in a similar way to calculating the speed of moving objects:

wave space speed space equals space distance over time

  • Where:

    • Wave speed is measured in metres per second (m/s)

    • Distance travelled by the wave is measured in metres (m)

    • Time taken is measured in seconds (s)

  • All waves obey the wave equation, which is another way to calculate the wave speed:

table row cell nu space end cell equals cell space f space cross times space lambda end cell end table

  • Where:

    • nu = wave speed in metres per second (m/s)

    • f = frequency in hertz (Hz)

    • lambda = wavelength in metres (m)

  • You may need to rearrange the wave equation, which can be done using a formula triangle

Formula triangle relating wave speed, frequency and wavelength

Formula triangle with wave speed on the top, frequency and wavelength on the bottom
Simply cover up the variable you want to calculate and the formula triangle will show you the correct structure of the equation

Worked Example

Sound waves in air travel with a frequency of 170 Hz and a wavelength of 2 m.

Calculate the speed of sound waves in air.

Answer:

Step 1: List the known quantities

  • Frequency, f space equals space 170 space Hz

  • Wavelength, lambda space equals space 2 space straight m

Step 2: Write out the wave equation

nu space equals space f space cross times space lambda

Step 3: Substitute the known quantities into the wave equation  to calculate wave speed

table row cell nu space end cell equals cell space 170 space cross times space 2 end cell row blank blank blank row cell nu space end cell equals cell space 340 space straight m divided by straight s end cell end table

Worked Example

A wave in a pond has a speed of 0.15 m/s and a frequency of 0.5 Hz.

Calculate the wavelength of the wave.

Answer:

Step 1: List the known quantities

  • Wave speed, nu space equals space 0.15 space straight m divided by straight s 

  • Frequency, f space equals space 0.5 space Hz 

Step 2: Write out the wave equation

nu space equals space f space cross times space lambda

Step 3: Rearrange the equation to make wavelength the subject

lambda space equals fraction numerator space v over denominator f end fraction

Step 4: Substitute the known values to calculate the wavelength

table row cell lambda space end cell equals cell space fraction numerator 0.15 over denominator 0.5 end fraction end cell row blank blank blank row cell lambda space end cell equals cell space 0.30 space straight m end cell end table

Exam Tip

When stating equations make sure you use the right letters:

  • For example, use lambda for wavelength, not L or W

  • If you can’t remember the correct letters, then just state the word equations

Be careful with units:

  • Wavelength is usually measured in metres and speed in m/s, but sometimes wavelength may be given in other units, such as cm or km. When calculating wave speed, check what units for speed the question requires and be prepared to convert the wavelength accordingly.

  • Likewise, watch out for frequency given in kHz: 1 kHz = 1000 Hz

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    Leander Oates

    Author: Leander Oates

    Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.