The Doppler Shift

The spectral lines of atoms and molecules have well measured wavelengths.  For example the red line of hydrogen is at a wavelength of 0.656285 mm

• 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 upon the object's motion toward or away from the observer.<1i>
• Quantitatively

The lower spectrum is shifted toward longer wavelength (Red shifted),
indicating that the source is moving away from the observer.


Important Points

• 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.''
• Receding objects produce a "redshift.''
• The greater the velocity, the greater the shift.

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Last Updated: 12/2/98