The Doppler Shift

• The spectral lines of atoms and molecules have well measured wavelengths. For example the red emission line of hydrogen is at a wavelength of 0.656285 m.
  • This laboratory wavelength is called the rest  wavelength.

• Light observed coming from an object moving toward or away from the observer will have its wavelength shifted from the rest wavelength.

  • The amount of the shift will depend on the speed of the object.

  • Light coming from a source moving away from the observer will be shifted from the rest wavelength toward longer wavelengths (i.e. toward the red for visible light redshift)

  • If the source of light is moving toward the observer the wavelength will be shifted to shorter wavelengths (blueshift ).

• The amount of shift depends only on the object's motion toward or away from the observer (radial velocity). An object moving across the observers line of sight will produce no shift in wavelength.
  • Doppler shifts measure radial velocity only.

• Quantitatively -- larger velocities produce larger shifts

  

• For example the red line of hydrogen, which has a rest wavelength of 0.656285 m ( ) is observed in the spectrum of the star Vega at 0.656255 m ( ).

  

  

Important Points about the Doppler Shift

• Used to measure the velocities of stars and galaxies.

• Only the component of motion toward or away from the observer shifts the wavelengths of the spectral lines.

• Approaching objects produce a ``blueshift.''
  • a shift toward shorter wavelengths.

• Receding object produce a ``redshift.''
  • a shift toward longer wavelengths.

• The greater the velocity the greater the shift.

Notes

Updated February 20, 2002