Astrophysics and Cosmology

1
Doppler Shift
Astrophysics and Cosmology
Longer ?, lower f
Shorter ?, higher f
The Wavelengths get squished in the direction
of motion, and stretched in the opposite
direction.
2
Doppler Shift
Astrophysics and Cosmology
Longer ?, lower f
Shorter ?, higher f
In the visible spectrum, the color red is
associated with the longer wavelength end of the
visible window. The stretching of the wave to a
longer wavelength is called a red shift
3
Doppler Shift
Astrophysics and Cosmology
Longer ?, lower f
Shorter ?, higher f
In the visible spectrum, the color blue is
associated with the longer wavelength end of the
visible window. The shortening of the wave to a
shorter wavelength is called a blue shift
4
Doppler Shift
Astrophysics and Cosmology
A train moves toward an observer at a speed of 25
m/sec. It blows its whistle, and also has its
front light on. The observer hears the whistle
and sees the light. The wavelength of the sound
wave emitted is .3 meters and the speed of sound
is 343 m/sec, and the wavelength of the light is
500 nm. What is the Doppler shifted wavelength
of the sound wave?
The Doppler shifted wavelength is given by
Apparent Wavelength
Velocity of Source

1
True Wavelength
Wave Speed
Since the train is moving toward the observer,
the velocity of the source is negative.
Therefore, for the sound wave ?Apparent / ?True
?Apparent / .3 m 1 (25 m/sec / 343 m/sec)
? ?Apparent (.92) (.3 m) .278 m. This is an
8 shift in the wavelength of the sound wave,
which would be noticeable.
5
Doppler Shift
Astrophysics and Cosmology
A train moves toward an observer at a speed of 25
m/sec. It blows its whistle, and also has its
front light on. The observer hears the whistle
and sees the light. The wavelength of the sound
wave emitted is .3 meters and the speed of sound
is 343 m/sec, and the wavelength of the light is
500 nm. What is the frequency of the sound
wave produced by the source? What is the
frequency of the Doppler shifted sound wave?
Using data from the previous slide For the wave
at the source, f v/?True (343 m/sec) / (0.3
m) 1,143 Hz. For the Doppler Shifted wave, f
v/?Apparent (343 m/sec) / (0.278 m) 1,234 Hz.
6
Doppler Shift
Astrophysics and Cosmology
A train moves toward an observer at a speed of 25
m/sec. It blows its whistle, and also has its
front light on. The observer hears the whistle
and sees the light. The wavelength of the sound
wave emitted is .3 meters and the speed of sound
is 343 m/sec, and the wavelength of the light is
500 nm. What is the Doppler shifted wavelength
of the light?
The Doppler shifted wavelength is given by
Apparent Wavelength
Velocity of Source

1
True Wavelength
Wave Speed
Since the train is moving toward the observer,
the velocity of the source is negative.
Therefore, for the light, A?Apparent / ?True
?Apparent / 500 x 10-9 m 1 (25 m/sec / 3 x
108 m/sec) ? ?Apparent 500 x 10-9 m. The
shift is so small for light, the change in
wavelength would not be noticeable.
7
Doppler Shift
Astrophysics and Cosmology
A train moves toward an observer at a speed of 25
m/sec. It blows its whistle, and also has its
front light on. The observer hears the whistle
and sees the light. The wavelength of the sound
wave emitted is .3 meters and the speed of sound
is 343 m/sec, and the wavelength of the light is
500 nm. What is the frequency of the light
wave produced by the source? What is the
frequency of the Doppler shifted light wave?
Using data from the previous slide For the wave
at the source, f c/?True (3 x 108 m/sec) /
(500 x 10-9 m) 6 x 1014 Hz. For the Doppler
Shifted wave, f c/?Apparent (3 x 108 m/sec) /
(500 x 10-9 m) 6 x 1014 Hz. Because the
Doppler shift is such a small effect for the
light, the change in frequenci is insignificant.
8
Doppler Shift
Astrophysics and Cosmology
A train moves toward an observer at a speed of 25
m/sec. It blows its whistle, and also has its
front light on. The observer hears the whistle
and sees the light. The wavelength of the sound
wave emitted is .3 meters and the speed of sound
is 343 m/sec, and the wavelength of the light is
500 nm. What is the Doppler shifted wavelength
of the sound wave? What is the Doppler shifted
wavelength of the light?
The Doppler shifted wavelengths are given by
Apparent Wavelength
Velocity of Source

1
True Wavelength
Wave Speed
In order for a Doppler shift to me noticeable,
the speed of the source must be comparable to the
speed of the wave. Sound waves do not travel very
fast, and therefore Doppler shifts for sound
waves are quite noticeable for fast moving cars
or trains. Since EM waves move so fast, objects
on the earth cannot move fast enough to produce a
noticeable Doppler shift in EM waves.
Astronomical objects can, in fact, move at speeds
high enough to detect a Doppler Shift in EM
waves.