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

Nucleus contains Protons and Neutrons

“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

Nucleus

Emission

Photons (light-waves) can also be absorbed by an atom when an electron moves from a lower energy level to a

higher energy level

Nucleus

Absorption

€

E = hf =hcλ

Hydrogen

0 eV

10.19 eV

12.07 eV 12.73 eV

13.6 eV (binding energy)

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

• A star’s chemical composition - peak wavelength of the spectral curve

- 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.”

•  When something which is giving off light moves towards or away from you, the wavelength of the emitted light is changed or shifted

Doppler Effect

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

• A star’s chemical composition - peak wavelength of the spectral curve

- dips in the spectral curve or the lines in the absorption spectrum

•  A star’s motion - Doppler shift

23

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

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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.

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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

What we can Learn about Stars. 1!

Object A

Wavelength

visible range

Ener

gy O

utpu

t per

seco

nd

Object D

Wavelength V I B G Y O R

visible range

Ener

gy O

utpu

t per

seco

nd

Object C

Wavelength V I B G Y O R

visible range

Ener

gy O

utpu

t per

seco

nd

Object B

Wavelength V I B G Y O R

visible range

Ener

gy O

utpu

t per

seco

nd

V I B G Y O R

Put these stars in order from smallest to largest (use = signs if necessary)!

Rank the stars A-E from largest to smallest. !

Temperature (K)

Lum

inos

ity (s

olar

uni

ts)

A

D 10,000

100

10

1

.1

.01

.001

.0001

1,000

C B

E

20000! 10000! 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!