The Doppler Effect (Oxford AQA IGCSE Physics)

The Doppler Effect

• The Doppler effect is defined as:

If a wave source is moving relative to an observer there will be a change in the observed wavelength and frequency

• For a source moving relative to an observer, this effect can be observed for any type of wave, including:

  • Light waves

  • Sound waves

  • Microwaves

• Examples of this effect being observed include:

  • A change in the wavelength of light emitted by distant galaxies (red shift)

  • The pitch of a police siren changes as the car drives past

Explaining the Doppler effect

• Usually, when a stationary object emits waves, the waves spread out symmetrically

Stationary source of sound waves

• If the object emitting waves begins to move, the waves become condensed together in front of it and stretched behind it

Stationary and moving sources

• When the source moves away from the observer, the observed wavelength increases and the frequency decreases

• When the source moves towards the observer, the observed wavelength decreases and the frequency increases

Position of a stationary observer