Download - 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.

The Doppler EffectThe Doppler Effect

The Doppler EffectThe Doppler Effect

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

The Doppler Effect and SoundThe Doppler Effect and Sound

Stationary source and stationary observerStationary source and stationary observer

s o


Moving source and stationary observerMoving source and stationary observer

s o

High frequency, short wavelengthLow frequency,

long wavelength

Sonic BoomsSonic Booms

Caused when an airplane travels faster Caused when an airplane travels faster than sound.than sound. Supersonic = faster than soundSupersonic = faster than sound

Sound waves “pile up” in the direction of Sound waves “pile up” in the direction of travel.travel. Form a high pressure cone.Form a high pressure cone. Stationary observers hear a loud boom when Stationary observers hear a loud boom when

the front of the cone passes by.the front of the cone passes by.

Supersonic FlightSupersonic 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 ShuttleThe Space Shuttle

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

The Space ShuttleThe Space Shuttle

The Doppler Effect and LightThe Doppler Effect and Light

Light is also subject to the Doppler effect.Light is also subject to the Doppler effect. Astronomers use the Doppler shift in starlight Astronomers use the Doppler shift in starlight

to determine if stars are moving toward or to determine if stars are moving toward or away from Earth.away from Earth.

Stars moving toward the Earth have their light Stars moving toward the Earth have their light wavelengths shortened.wavelengths shortened.

Causes the light to shift toward the blue end of the Causes the light to shift toward the blue end of the visible spectrum.visible spectrum.

Stars moving away from Earth have their light Stars moving away from Earth have their light wavelengths lengthened.wavelengths lengthened.

Causes the light to shift toward the red end of the Causes the light to shift toward the red end of the visible spectrum.visible spectrum.

Red Shifting and Blue ShiftingRed Shifting and Blue Shifting

The light given off by a star The light given off by a star contains “gaps” called contains “gaps” called absorption lines.absorption lines. Absorption lines correspond to Absorption lines correspond to

the elements that make up a the elements that make up a star.star.

Every element gives its own Every element gives its own absorption signature to the star’s absorption signature to the star’s light.light.

If we know what a star is made If we know what a star is made of, we can usually predict where of, we can usually predict where the absorption lines should be in the absorption lines should be in the star’s spectrum of light.the star’s spectrum of light.


If the star is moving towards If the star is moving towards Earth, its light gets “bunched Earth, its light gets “bunched up” and shortened.up” and shortened. The blue-violet end of the The blue-violet end of the

spectrum contains the shortest spectrum contains the shortest wavelengths.wavelengths.

The incoming light from the The incoming light from the star appears “bluer” than it star appears “bluer” than it would if the star were would if the star were stationary relative to Earth.stationary relative to Earth. All of the expected absorption All of the expected absorption

lines get pushed toward the lines get pushed toward the blue-violet end of the spectrum.blue-violet end of the spectrum.


If the star is moving away from If the star is moving away from Earth, its light gets “stretched Earth, its light gets “stretched out” and lengthened.out” and lengthened. The red end of the spectrum The red end of the spectrum

contains the longest contains the longest wavelengths.wavelengths.

The incoming light from the The incoming light from the star appears “redder” than it star appears “redder” than it would if the star were would if the star were stationary relative to Earth.stationary relative to Earth. All of the expected absorption All of the expected absorption

lines get pushed toward the red lines get pushed toward the red end of the spectrum.end of the spectrum.


The pair of spectra at left The pair of spectra at left represent the expected represent the expected absorption spectrum of a absorption spectrum of a stationary star (left) and the stationary star (left) and the measured spectrum of a star measured spectrum of a star that is moving relative to Earth that is moving relative to Earth (right).(right). Can you tell if the star is moving Can you tell if the star is moving

toward or away from the Earth?toward or away from the Earth?

The Doppler Effect and the Big BangThe Doppler Effect and the Big Bang

In 1912 an astronomer named Vesto In 1912 an astronomer named Vesto Slipher measured a red shift in the light Slipher measured a red shift in the light coming from a distant galaxy.coming from a distant galaxy. This indicated that the galaxy was moving This indicated that the galaxy was moving

away from Earth.away from Earth.

He later discovered that the light from He later discovered that the light from almost almost allall distant galaxies was red shifted. distant galaxies was red shifted.Edwin Hubble later showed that the farther Edwin Hubble later showed that the farther away the galaxies are, the faster they away the galaxies are, the faster they seem to be receding from us.seem to be receding from us.

The Doppler Effect and the Big BangThe Doppler Effect and the Big Bang

In 1927 a Belgian physicist and Roman In 1927 a Belgian physicist and Roman Catholic priest named Georges Lemaître Catholic priest named Georges Lemaître proposed that the galaxies were receding proposed that the galaxies were receding because the universe was expanding.because the universe was expanding. Four years later he would propose the first Four years later he would propose the first

model of the Big Bang, in which the Universe model of the Big Bang, in which the Universe expanded from an infinitely hot, dense state expanded from an infinitely hot, dense state some time in the distant past.some time in the distant past.

Later evidence has supported this model.Later evidence has supported this model.

The Big BangThe 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.