Doppler Shift (Edexcel A Level Physics)

• 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

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