Cosmology: (e) Doppler effect; Doppler shift of electromagnetic radiation

The idea

The Doppler effect is the change in observed frequency and wavelength when a source of waves moves relative to an observer. For electromagnetic radiation, a source moving away from the observer has its observed wavelength stretched, producing a red shift; a source moving towards the observer has its wavelength compressed, producing a blue shift. The size of the shift depends on the relative speed along the line of sight. In cosmology, nearly all distant galaxies show red shift, which is evidence that they are receding. You should be able to identify the direction of motion from the sign of the shift and distinguish red shift from blue shift in spectra.

What earns the marks

• Doppler effect is the change in observed frequency or wavelength due to relative motion between source and observer.
• For electromagnetic radiation, motion away from the observer increases observed wavelength (red shift).
• Motion towards the observer decreases observed wavelength (blue shift).
• The shift depends on the component of relative velocity along the line of sight.
• Red shift of galactic spectra is evidence for recession of galaxies.

Common mistakes and how to put them right

• Thinking red shift means the galaxy is actually red in colour.

  Put it right: Red shift refers to the position of spectral lines shifted towards longer wavelengths.

• Confusing the direction of shift.

  Put it right: Away means red shift, towards means blue shift.

• Assuming the Doppler shift depends on distance rather than relative velocity.

  Put it right: The shift depends on the relative speed along the line of sight.

• Believing the Doppler effect only applies to sound.

  Put it right: It applies to all waves, including electromagnetic radiation.

Exam tips

• Read the question carefully to identify whether the source is moving towards or away from the observer.
• Remember that red shift means longer wavelength and lower frequency; blue shift means shorter wavelength and higher frequency.
• In multiple-choice questions, eliminate options that confuse red shift with blue shift or that link shift directly to distance.