Doppler Shift
October 19, 2009
Taking Care of Business (TCB)
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Read textbook Units 25 and 26
No Homework next week!
Moon Observations – Monday, October 26
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Need total of 9 observations, all columns filled.
Test #2 – Friday, October 16 to Monday
October 19
 Reserve a test time!!!!
What can we learn by
analyzing starlight?
• A star’s temperature
- peak wavelength of the spectral curve
• A star’s chemical composition
- dips in the spectral curve or the
lines in the absorption spectrum
• A star’s motion
•Moving towards us/ moving away
•How fast the star is going
The Doppler Effect
• Definition: “The change in wavelength of
radiation (light) due to the relative motion
between the source and the observer along
the line of sight.”
Astronomers use the Doppler Effect to learn
about the radial (along the line of sight) motions
of stars, and other astronomical objects.
On a boat, in
the front you
can see shorter
waves
Boat, in the
back the waves
are longer
Real Life Examples of Doppler
Effect (Sound or Light)
• Sound, NOT the volume of the sound, it’s a build
of sound waves (pitch change)
• Trains, airplanes, moving vehicles, police sirens,
bats
• Car headlights, radar gun
Doppler Effect
• When something which is giving off light moves
towards or away from you, the wavelength of the
emitted light is changed or shifted
V=0
Doppler Effect
• When something which is giving off light moves
towards or away from you, the wavelength of the
emitted light is changed or shifted
V=0
longer
shorter
Doppler Effect
• When the source of light is moving away from the
observer the wavelength of the emitted light will
appear to increase. We call this a “redshift”.
Red because the wave
length is the longest.
Doppler Effect
• When the source of light is moving towards the
observer the wavelength of the emitted light will
appear to decrease. We call this a “blueshift”.
Blue, shifted
towards the blue
Astronomy Application
V=0
Doppler Shifts
• Redshift (to longer wavelengths): The source is
moving away from the observer
• Blueshift (to shorter wavelengths): The source is
moving towards the observer
Dl v

l0 c
(700-400)/700 = V/3x10^8 m/s
V= 34% C
Dl = wavelength shift
lo = wavelength if source is not moving, 700 nm
v = velocity of source
c = speed of light
The Doppler Effect causes light
from a source moving away to:
1. be shifted to shorter wavelengths.
2. be shifted to longer wavelengths.
3. change in velocity. (In space, light will
travel in a constant speed.)
4. Both a and c above
5. Both b and c above
You observe two spectra (shown below) that are redshifted
relative to that of a stationary source of light. Which of the
following statements best describes how the sources of light
that produced the two spectra were moving?
BLUE
RED
Spectrum A
Spectrum B
1. Source A is moving faster than source B.
2. Source B is moving faster than source A., we are
told it is redshifted!
3. Both sources are moving with the same speed.
4. It is impossible to tell from looking at these spectra.
A bright star is moving toward Earth. If you
were to look at the spectrum of this star,
what would it look like?
1. an absorption spectrum that is redshifted relative to an
unmoving star
2. an emission spectrum that is redshifted relative to an
unmoving star
3. a continuous spectrum that is blueshifted relative to an
unmoving star
4. an absorption spectrum that is blueshifted relative to
an unmoving star, blueshifted (moving towards us),
absorption
5. a continuous spectrum that is redshifted relative to an
Links to In-Class Problems
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Go to astro.unl.edu
Click on “Class Action” box on right.
Light: #11, #12, #13