The Doppler Effect (Edexcel IGCSE Physics)

• The Doppler Effect is defined as:

The apparent change in wavelength and frequency of a wave emitted by a moving source

• This effect can be observed in a variety of scenarios whenever sources of waves move:
  • The frequency of the sound waves emitted by ambulance or police sirens goes from a high pitch to a low pitch as the vehicle whizzes past
  • Galaxies in outer space emit electromagnetic radiation - light waves - which appear to be redder than normal because the stars are moving away from us

Explaining the Doppler Effect

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

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

• If the object emitting waves begins to move, the waves can get squashed together at one end of the object, and stretched at the other end

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

• The waves at the front of the moving vehicle appear to be squashed together:
  • This means the wavelength decreases (and the frequency of the waves increases)

• The waves behind the moving vehicle appear to be stretched out:
  • This means the wavelength increases (and the frequency of the waves decreases)