The Doppler Effect

Doppler Effect – Change in frequency and wavelength of a wave for an observer moving relative to the source of the wave. Source is moving and observer is stationary. Source is moving and observer is stationary. Source is stationary and observer is moving. Source is stationary and observer is moving. Source and observer are both moving. Source and observer are both moving. Named for Austrian physicist Christian Doppler. Proposed the effect in Proposed the effect in 1842.

The Doppler Effect and Sound Stationary source and stationary observer so

The Doppler Effect and Sound Moving source and stationary observer so High frequency, short wavelength Low frequency, long wavelength

The Doppler Effect and Sound

Sonic Booms Caused when an airplane travels faster than sound. Supersonic = faster than sound Supersonic = faster than sound Sound waves “pile up” in the direction of travel. Form a high pressure cone. Form a high pressure cone. Stationary observers hear a loud boom when the front of the cone passes by. Stationary observers hear a loud boom when the front of the cone passes by.

Supersonic Flight A US Navy F/A-18 Super Hornet is shown here breaking the sound barrier. There is an invisible high pressure cone starting at the nose of the aircraft. The cone-shaped cloud is formed when water vapor condenses out of the air in the low- pressure wake of the high-pressure cone. This effect is called a Prandtl-Glauert singularity, or more simply, a vapor cone.

The Space Shuttle Aircraft in supersonic flight actually produce 2 booms: 1 from nose and 1 from tail. 1 from nose and 1 from tail. The following video allows you to hear the twin sonic booms produced by the space shuttle Atlantis as it approaches its Florida landing strip after re-entering Earth’s atmosphere in 2008.

The Space Shuttle

The Doppler Effect and Light Light is also subject to the Doppler effect. Astronomers use the Doppler shift in starlight to determine if stars are moving toward or away from Earth. Astronomers use the Doppler shift in starlight to determine if stars are moving toward or away from Earth. Stars moving toward the Earth have their light wavelengths shortened. Stars moving toward the Earth have their light wavelengths shortened. Causes the light to shift toward the blue end of the visible spectrum. Stars moving away from Earth have their light wavelengths lengthened. Stars moving away from Earth have their light wavelengths lengthened. Causes the light to shift toward the red end of the visible spectrum.

Red Shifting and Blue Shifting The light given off by a star contains “gaps” called absorption lines. Absorption lines correspond to the elements that make up a star. Absorption lines correspond to the elements that make up a star. Every element gives its own absorption signature to the star’s light. Every element gives its own absorption signature to the star’s light. If we know what a star is made of, we can usually predict where the absorption lines should be in the star’s spectrum of light. If we know what a star is made of, we can usually predict where the absorption lines should be in the star’s spectrum of light.

Red Shifting and Blue Shifting If the star is moving towards Earth, its light gets “bunched up” and shortened. The blue-violet end of the spectrum contains the shortest wavelengths. The blue-violet end of the spectrum contains the shortest wavelengths. The incoming light from the star appears “bluer” than it would if the star were stationary relative to Earth. All of the expected absorption lines get pushed toward the blue-violet end of the spectrum. All of the expected absorption lines get pushed toward the blue-violet end of the spectrum.

Red Shifting and Blue Shifting If the star is moving away from Earth, its light gets “stretched out” and lengthened. The red end of the spectrum contains the longest wavelengths. The red end of the spectrum contains the longest wavelengths. The incoming light from the star appears “redder” than it would if the star were stationary relative to Earth. All of the expected absorption lines get pushed toward the red end of the spectrum. All of the expected absorption lines get pushed toward the red end of the spectrum.

Red Shifting and Blue Shifting The pair of spectra at left represent the expected absorption spectrum of a stationary star (left) and the measured spectrum of a star that is moving relative to Earth (right). Can you tell if the star is moving toward or away from the Earth? Can you tell if the star is moving toward or away from the Earth?

The Doppler Effect and the Big Bang In 1912 an astronomer named Vesto Slipher measured a red shift in the light coming from a distant galaxy. This indicated that the galaxy was moving away from Earth. This indicated that the galaxy was moving away from Earth. He later discovered that the light from almost all distant galaxies was red shifted. Edwin Hubble later showed that the farther away the galaxies are, the faster they seem to be receding from us.

The Doppler Effect and the Big Bang In 1927 a Belgian physicist and Roman Catholic priest named Georges Lemaître proposed that the galaxies were receding because the universe was expanding. Four years later he would propose the first model of the Big Bang, in which the Universe expanded from an infinitely hot, dense state some time in the distant past. Four years later he would propose the first model of the Big Bang, in which the Universe expanded from an infinitely hot, dense state some time in the distant past. Later evidence has supported this model.

The Big Bang Light traveling between distant galaxies gets “stretched out” (red-shifted) as the space between the galaxies expands. The individual stars in galaxies don’t move apart because they are held close together by gravity.