CIE AS Physics

Revision Notes

ASPhysicsCIERevision Notes7. Waves7.1 Waves: Transverse & Longitudinal7.1.3 The Wave Equation

Syllabus Edition

First teaching 2020

Last exams 2024

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7.1.3 The Wave Equation

Test Yourself

Derivation of v = f λ

  • Using the definitions of speed, frequency and wavelength, the wave equation v = fλ can be derived
  • This is an important relationship between three key properties of a wave
  • The derivation for this is shown below

Derivation of v=fλ (1), downloadable AS & A Level Physics revision notesDerivation of v=fλ (2), downloadable AS & A Level Physics revision notes

Derivation of v = fλ

 

Exam Tip

You will be expected to remember all the steps for this derivation (but do not need to write the full definition for each variable). If you are unsure as to where speed = distance/time comes from, make sure to revisit chapter “2. Kinematics”.

The Wave Equation

  • The wave equation links the speed, frequency and wavelength of a wave
  • This is relevant for both transverse and longitudinal waves

 Wave equation, downloadable AS & A Level Physics revision notes

The Wave Equation

  • The wave equation tells us that for a wave of constant speed:
    • As the wavelength increases, the frequency decreases
    • As the wavelength decreases, the frequency increases

 Frequency and wavelength, downloadable AS & A Level Physics revision notes

The relationship between frequency and wavelength of a wave

Worked example

The wave in the diagram below has a speed of 340 m s–1.WE - Wave equation question image, downloadable AS & A Level Physics revision notes

What is the wavelength of the wave?

Worked example - wave equation, downloadable AS & A Level Physics revision notes

 

Exam Tip

You may also see the wave equation be written as c = fλ where c is the wave speed. However, c is often used to represent a specific speed ー the speed of light ( 3 x 108 ms-1). Only electromagnetic waves travel at this speed, therefore it’s best practice to use v for any speed that isn’t the speed of light instead.

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