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Doppler Shift
Part I. Motion of Source
Because of the Doppler effect, light emitted by an object can appear to change
wavelength due to its motion toward or away from an observer. When the observer and
the source of light are moving toward each other the light is shifted to shorter
wavelengths (blueshifted). When the observer and the source of light are moving away
from each other the light is shifted to longer wavelengths (redshifted).
a)
Star is not moving
b)
c)
d)
1. In which of the situations shown (a – d) will the observer receive light that is shifted
to shorter wavelengths? Explain your reasoning.
2. In which of the situations shown (a – d) will the observer receive light that is shifted
to longer wavelengths? Explain your reasoning.
©CAPER TEAM
DRAFT EDITION, 2005
LECTURE-TUTORIALS FOR INTRODUCTORY ASTRONOMY
1
Doppler Shift
3. In which of the situations shown (a – d) will the observer receive light that is not
shifted at all? Explain your reasoning.
Part II. Shift in Absorption Spectra
When we study an astronomical object like a star or galaxy, we usually examine the
spectrum of light it gives off. Recall that the spectrum of an object contains lines that
work like fingerprints to help identify different elements. Since the lines of a spectrum
occur at specific wavelengths we can determine that an object is moving when we see
that the lines have been shifted to either longer or shorter wavelengths. For the
absorption line spectra shown below, blue light is shown on the left-hand side and red
light is shown on the right-hand side.
Blue
Red
a)
b)
c)
For the three absorption line spectra shown above (a, b and c), one of the spectra
corresponds to a star that is not moving relative to you, one spectra is from a star that is
moving towards you and one spectra is from a star that is moving away from you.
4. Which of the three spectra, corresponds with the star moving toward you? Explain
your reasoning.
©CAPER TEAM
DRAFT EDITION, 2005
LECTURE-TUTORIALS FOR INTRODUCTORY ASTRONOMY
2
Doppler Shift
5. Which of the three spectra, corresponds with the star moving away from you?
Explain your reasoning.
Part III. Size of Shift and Speed
If two sources of light are moving relative to an observer, the star that is moving faster
will appear to undergo a greater Doppler Shift.
Consider the four spectra below. The spectrum labeled (c) is an absorption line spectrum
from a star that is at rest. Again, note that blue light is shown on the left-hand side of
each spectrum and red light is shown on the right-hand side of each spectrum.
Blue
Red
a
b
c
d
6. Which of the four spectra would be from the star that is moving the fastest? Would
this star be moving toward or away form the observer?
7. Which of the four spectra would be from the star that is moving the slowest?
Describe the motion of this star.
©CAPER TEAM
DRAFT EDITION, 2005
LECTURE-TUTORIALS FOR INTRODUCTORY ASTRONOMY
3
Doppler Shift
8. You overhear two students discussing the topic of Doppler shifts.
Student 1: Since Betelgeuse is a red star, it must be going away from us, and since Rigel
is a blue star it must be coming towards us.
Student 2: I disagree, the color of the star does not tell you if it is moving. You have to
look at the shift in wavelength of a star’s spectral lines to determine whether it’s moving
towards or away from you.
Do you agree with Student 1, Student 2, neither or both? Why or why not?
©CAPER TEAM
DRAFT EDITION, 2005
LECTURE-TUTORIALS FOR INTRODUCTORY ASTRONOMY
4
Exercise #1
Astronomy Ranking Task:
Doppler Shift
Description: The figure below shows a train traveling toward the right and sounding its horn.
Three persons are shown at locations A, B, and C. Assume that all three people can hear the
train blowing its horn.
A. Ranking Instructions: Rank the pitch of the horn from highest pitch (or frequency) to
lowest pitch (or frequency) as heard by each person (A – C)
Ranking Order: Highest 1 _______ 2 _______ 3 _______ Lowest
Or, the pitch heard by each person would be the same. ______ (indicate with check mark).
Carefully explain your reasoning for ranking this way:
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
B. Ranking Instructions: Rank the wavelength (from longest to shortest) of the sound of the
horn as heard by each person (A – C).
Ranking Order: Longest 1 _______ 2 _______ 3 _______ Shortest
Or, the wavelength heard is the same for each person.____ (indicate with check mark).
Carefully explain your reasoning for ranking this way:
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Copyright © 2005 Conceptual Astronomy and Physics Education Research (CAPER) Team
University of Arizona
Exercise #2
Astronomy Ranking Task:
Doppler Shift
Description: The figure below shows the motion of five distant stars (A - E) relative to a
stationary observer (telescope). The speed and direction of each star is indicated by the length
and direction of the arrows shown.
C
E
A
B
D
Observer
Distant Stars
Ranking Instructions: Rank the Doppler shift of the light observed from each star (A – E) from
greatest “blueshift”, through no shift, to greatest “redshift”.
Ranking Order:
Greatest blueshift 1_______ 2 _______ 3 _______ 4 _______ 5 ________ Greatest redshift
Or, the Doppler shift for each star is the same. ______ (indicate with check mark).
Carefully explain your reasoning for ranking this way:
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
Copyright © 2005 Conceptual Astronomy and Physics Education Research (CAPER) Team
University of Arizona
Astronomy Ranking Task:
Doppler Shift
Exercise #3
Description: The first spectra shown below is of an element as it appears in a laboratory here on
Earth. In addition, the spectra of five stars (A - E) as seen from Earth are shown. Assume that the
left end of each spectrum corresponds to shorter wavelengths (blue light) and that the right end of each
spectrum corresponds with longer wavelengths (red light).
Blue
Red
Lab Spectra
Star A
Star B
Star C
Star D
Star E
A. Ranking instructions: Rank the size of the Doppler shift (from largest to smallest) for the
light from each star (A – E).
Ranking Order: Largest 1 ____ 2 ____ 3 ____ 4 _____ 5 _____ Smallest
Or, the Doppler shift of the light from the stars would all be the same. _____ (indicate with a
check mark)
Carefully explain your reasoning for ranking this way:
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Copyright © 2005 Conceptual Astronomy and Physics Education Research (CAPER) Team
University of Arizona
B. Ranking instructions: Rank the speed of the stars (A – E) from moving fastest toward the
Earth to moving fastest away from Earth.
Ranking Order:
Moving fastest toward 1 ____ 2 ____ 3 ____ 4 _____ 5 _____ Moving fastest away
Or, all the stars have the same speed. _____ (indicate with a check mark)
Carefully explain your reasoning for ranking this way:
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Copyright © 2005 Conceptual Astronomy and Physics Education Research (CAPER) Team
University of Arizona
Astronomy Ranking Task:
Doppler Shift
Exercise #4
Description: An important line in the absorption spectrum of stars occurs at a wavelength of
656nm for stars at rest. Imagine that you study five stars (A-E) from Earth and discover that this
absorption line is observed at the wavelength shown in the table below for each of the five stars.
STAR
A
B
C
D
E
Observed Wavelength
of Absorption line
650 nm
663 nm
656 nm
657 nm
646 nm
A. Ranking instructions: Rank the size of the Doppler shift (from largest to smallest) observed
tonight for the light from each star (A – E).
Ranking Order: Largest 1 ____ 2 ____ 3 ____ 4 _____ 5 _____ Smallest
Or, the Doppler shift of the light from the stars would all be the same. _____ (indicate with a
check mark)
Carefully explain your reasoning for ranking this way:
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
B. Ranking instructions: As observed tonight, rank the speed of the stars (A – E) from moving
fastest toward the Earth, through not moving at all, to moving fastest away from Earth.
Ranking Order:
Moving fastest toward 1 ____ 2 ____ 3 ____ 4 _____ 5 _____ Moving fastest away
Or, all the stars would have the same speed _____ (indicate with a check mark)
Carefully explain your reasoning for ranking this way:
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Copyright © 2005 Conceptual Astronomy and Physics Education Research (CAPER) Team
University of Arizona