The Doppler Shift
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The Doppler Shift
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The wave appears shifted in
frequency from the original,
because of the speed of the
object from which it was
emitted or reflected.
An increase in frequency
shows an object is moving
toward you.
A decrease in frequency
shows an object is moving
away from you.
Click here to see a Doppler
shift video
image is courtesy of Windows to the Universe, http://www.windows.ucar.edu.
Astronomy and the Doppler Shift
• Astronomers can measure the
Doppler Shift of a moving
object.
• The radio wave has a known
frequency for stationary
objects.
• If target is moving away,
waves in the signal will be
stretched, and will have a
lower frequency.
• If the target is moving closer,
waves will be compressed and
will have a higher frequency.
• For more information and a
cool video click:
DYK???
• Police radar guns transmit a
continuous wave. The radar gun
compares the frequency of the
wave reflected (the echo) from
the moving vehicle to the
frequency of the transmitted
wave from the gun, and displays
the vehicle's computed speed.
Doppler Formula
The Doppler shift formula for a moving source:

2  1 v


1
1
c
where λ1 is the unshifted wavelength
λ2 is the shifted wavelength
c is the speed of light and
v is the velocity of the object emitting the light, relative to the
observer
Let’s Try An Example
• A spectral line normally at 4000 angstroms
is shifted to 3999 angstroms because of
the motion of the source. What is the
velocity of the source, along the line of
sight?
What Are We Given?
λ1 = 4000A
λ2 = 3999A
and c = 3 x 108 m/s
So
Solve for v:
 2  1 
  c
v  
 1 
= [(3999 – 4000)/4000]•3 x 108
= -7.5 x 104 m/s
Is the source coming towards us or away from us?
Towards us!
The Doppler shift is called a
“Red shift” if v > 0 (moving away) since the
wavelength is getting longer and the frequency
shorter
“Blue Shift” if v < 0 (moving closer) since the
wavelength is getting shorter and the frequency
higher
(recall that blue light is higher in frequency than red)
• To try another example click here.
• If not proceed to the next slide.
Uses of the Doppler Shift
• Radio astronomers can find the motion of
a source (stars, galaxies, gases, etc.) by
observing whether the emission or
absorption lines in its spectrum are shifted
in wavelength relative to their wavelengths
at rest
A typical passive radar system.
The range and Doppler shift can determined
Doppler Shift in Action
• A telescope in both the US and Japan will
have different velocities with respect to a
galaxy they are observing since the signal
received at each antenna is characterized
by a Doppler shift that depends on the
relative position and motion of the antenna
and the signal source