The Doppler Effect (Edexcel GCSE Physics): Revision Note

The Doppler Effect

• Usually, when an object emits waves, the wavefronts spread out symmetrically

  • If the wave source moves, the waves can become squashed together or stretched out

• Therefore, when a wave source moves relative to an observer there will be a change in the observed frequency and wavelength

Wavefronts are even in a stationary object but are squashed in the direction of the moving wave source

• A moving object will cause the wavelength, λ, (and frequency) of the waves to change:

  • The wavelength of the waves in front of the source decreases (λ – Δλ) and the frequency increases

  • The wavelength behind the source increases (λ + Δλ) and the frequency decreases

  • This effect is known as the Doppler effect

• Note: Δλ means 'change in wavelength'