Doppler Shift (Edexcel International A Level Physics)

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Doppler Shift

  • If a wave source is stationary, the wavefronts spread out symmetrically
  • If the wave source is moving, the waves can become squashed together or stretched out
    • If the wave source is moving towards an observer the wavefronts will appear squashed
    • If the wavefront is moving away from an observer the wavefronts will appear stretched out
  • Therefore, when a wave source moves relative to an observer there will be a change in the observed frequency and wavelength

doppler-effect, IGCSE & GCSE Physics revision notes

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
  • Note: Δλ means 'change in wavelength'
    • The actual wavelength emitted by the source remains the same
    • It is only the wavelength that is received by the observer that appears to have changed
  • This effect is known as the Doppler effect or Doppler shift

  • The Doppler effect is defined as:

      the apparent shift in wavelength occurring when the source of the waves is moving


  • The Doppler effect, or Doppler shift, can be observed using any form of electromagnetic radiation
  • It can be observed by comparing the light spectrum produced from a close object, such as our Sun, with that of a distant galaxy
    • The light from the distant galaxy is shifted towards the red end of the spectrum (There are more spectral lines in the red end)
    • This provides evidence that the universe is expanding

Comparing the light spectrum produced from the Sun and a distant galaxy, downloadable IGCSE & GCSE Physics revision notes

Comparing the light spectrum produced from the Sun and a distant galaxy

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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