The Doppler Effect (Edexcel IGCSE Physics: Double Science)

• The Doppler effect is defined as:

The apparent change in observed wavelength and frequency of a wave emitted by a moving source relative to an observer

• The Doppler effect can be observed whenever sources of waves move
  • The frequency of the sound waves emitted by ambulance or police sirens goes from a high pitch (high frequency) to a low pitch (low frequency) as the vehicle whizzes past 
  • Galaxies in outer space emit light waves which appear redder (longer wavelength) to an observer on Earth because the stars are moving away from us

Explaining the Doppler effect

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

A stationary source of sound waves

This stationary police car emits sound from the siren and the waves spread out symmetrically

• To an observer standing in front of an object moving towards them:
  
  • The waves appear to get squashed together because the wavelength appears to get shorter (and the frequency appears to get higher)
• To an observer standing behind an object moving away from them: 
  • The waves appear to get stretched apart because the wavelength appears to get longer (and the frequency appears to get lower)

The Doppler effect is observed when the source of the sound waves is moving

To an observer in front of the moving car, the wavelength appears smaller because they squash together. To an observer behind the moving car, the waves appear to stretch out