Download Light IV Doppler Shift

Light IV
Doppler Shift
February 14, 2012
Recap: Blackbody Curve!
The peak of the
blackbody curve tells
you temperature!
!
The overall height of
the blackbody curve
tells you Luminosity!
!
Luminosity depends
on Temperature and
Size!
Atom & Spectra
•  This is the spectrum
of the Sun
•  Dark features are
absorption lines
•  Tell us about
composition of the
Sun
•  The Sun has a hot
dense core
surrounded by a lower
density atmosphere
Nucleus contains
Protons and
Neutrons
Nucleus
Ground state is
lowest energy state
other states are called
excited states
Energy levels become
more closely spaced
further from the
nucleus
Photons (light-waves) are emitted
from an atom when an electron moves
from a higher energy level to a lower
energy level
Emission
Nucleus
Photons (light-waves) can also be
absorbed by an atom when an electron
moves from a lower energy level to a
higher energy level
Absorption
Nucleus
Hydrogen
13.6 eV (binding energy)
12.73 eV
12.07 eV
10.19 eV
0 eV
E = hf =
hc
λ
All stars
produce dark
line absorption
spectra
Time to Work on Lecture Tutorials
•  Work in pairs
•  Ask nearby groups for help
– Gabriels are in short supply today
•  Light & Atoms (pg 65, last class)
•  Analyzing Spectra (pg 71)
•  Types of Spectra (pg 63)
9
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
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.
http://www.youtube.com/watch?v=LIvVzJ6KZpk
Astronomers use the Doppler Effect to learn
about the radial (along the line of sight) motions
of stars, and other astronomical objects.
Real Life Examples of Doppler
Effect
•  Doppler Radar (for weather)
•  Airplane radar system (anything with radar)
•  Submarine sonar system
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.
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 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 .
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 .
The Doppler Effect
•  Definition: The change in wavelength of
radiation due to relative motion between the
source and the observer along the line of
sight.
Doppler Effect
•  Along the line of sight means the Doppler
Effect happens only if the object which is
emitting light is moving towards you or away
from you.
– An object moving side to side or perpendicular,
relative to your line of sight, will not experience a
Doppler Effect.
Astronomy Application
V=0
http://www.youtube.com/watch?v=gHxGz6RjZXs
Doppler effect of a spinning galaxy
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
Δλ = wavelength shift
λo = wavelength if source is not moving
v = velocity of source
c = speed of light
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
- Doppler shift
A bright star is moving away from Earth. Which of the
choices best completes the following statement describing
the spectrum of this star?
A(n) _________ spectrum that is __________ relative to
an unmoving star
A)  absorption; blueshifted
B)  emission; redshifted
C)  continuous; blueshifted
D)  absorption; redshifted
E)  continuous; redshifted
23
Doppler Shift
Lecture Tutorial p. 73
•  Work with a partner!
•  Read the instructions and questions carefully.
•  Discuss the concepts and your answers with one
another. Take time to understand it now!!!!
•  Come to a consensus answer you both agree on and
write complete thoughts into your LT.
•  If you get stuck or are not sure of your answer, ask
another group.
The Doppler Effect causes light
from a source moving away to:
1.  be shifted to shorter wavelengths.
2.  be shifted to longer wavelengths.
3.  changes in velocity.
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.
3.  Both sources are moving with the same speed.
4.  It is impossible to tell from looking at these spectra.
The following are NOT Think Pair
Questions, you are to do them by yourself pull out a piece of paper, name LEGIBLE
THINK OF THIS AS A QUIZ
27
visible
range
Object A
Energy Output per second
Energy Output per second
What we can Learn about Stars. 1!
V I B G Y O RWavelength
Energy Output per second
Energy Output per second
Object C
Object B
V I B G Y O RWavelength
V I B G Y O RWavelength
visible
range
visible
range
visible
range
Object D
V I B G Y O RWavelength
Put these stars in order from smallest to
largest (use = signs if necessary)!
10,000
Rank the stars A-E
from largest to
smallest. !
Luminosity (solar units)
D
1,000
100
C
10
1
B
A
.1
.01
E
.001
.0001
20000!
10000!
Temperature (K)
5000!
Describe what happens in each of the scenarios above
(i.e. emission/absorption of low/high energy, low/high
frequency, short/long wavelength)!
Use the four spectra for Objects A-D, shown below, to answer the
next question. Note that one of the spectra is from an object at rest
(not moving relative to Earth) and the remaining spectra come
from objects that are all moving away from the observer. [Assume
that the left end of the spectrum corresponds with short
wavelengths and the right end corresponds with long
wavelengths.]
Which object would be at rest?!
A)  Object A!
B)  Object B!
C)  Object C!
D)  Object D!
E) They are moving the same speed!