• 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