Download The Doppler Effect

The Doppler Effect
The Doppler Effect
Doppler Effect – Change in frequency
and wavelength of a wave for an observer
moving relative to the source of the wave.
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Source is moving and observer is stationary.
Source is stationary and observer is moving.
Source and observer are both moving.
Named for Austrian physicist Christian
Doppler.
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Proposed the effect in 1842.
The Doppler Effect and Sound
Stationary source and stationary observer
s
o
The Doppler Effect and Sound
Moving source and stationary observer
s
Low frequency,
long wavelength
o
High frequency,
short wavelength
The Doppler Effect and Sound
Sonic Booms
Caused when an airplane travels faster
than sound.
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Supersonic = faster than sound
Sound waves “pile up” in the direction of
travel.
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Form a high pressure cone.
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 lowpressure 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:
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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.
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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.
Causes the light to shift toward the blue end of the
visible spectrum.
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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.
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Absorption lines correspond to
the elements that make up a
star.
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.
Red Shifting and Blue Shifting
If the star is moving towards
Earth, its light gets “bunched
up” and shortened.
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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.
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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.
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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.
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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).
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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.
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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.
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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.