Doppler Effect How I Learned to Love Speed

What can you learn from spectra?

Temperature (energy)

Composition

Density

Speed

Moving toward or away

Demo: Sound

Demo: Car horn blaring as it passes

Car moving at nearly constant speed

Listen to pitch and volume.

The VOLUME of the horn:

1. sounded louder and louder as the source

approached and sounded fainter and fainter as

the source receded

2. stayed at a constant loudness as the source

approached then dropped to a fainter but

constant loudness as the source receded

3. stayed at the same constant loudness

throughout the motion

4. varied too much to tell what the volume was

doing

Note: The volume DOES increase on

approach and decrease on recession.

This is NOT the Doppler Effect.

The PITCH of the horn

1. became higher and higher as the source approached and became lower and lower as the source receded

2. stayed at a constant high pitch as the source approached and then dropped to a constant lower pitch as the source receded

3. stayed at the same constant pitch throughout the motion

4. stayed at the same pitch except at the moment the source passed

5. varied too much to tell what the pitch was doing

Volume

Time

Volume of car horn over time

Frequency

Time

Frequency of car horn over time

Doppler Effect

Shift in frequency (wavelength) due to

motion of source or observer or both.

Used to measure:

• Motion toward or away

• Speed

Sydney Harris

“I love hearing that lonesome wail of the train whistle as the frequency of the wave changes due to the Doppler Effect.”

Visual of waves from moving source:

http://www.acs.psu.edu/drussell/Demos/do

ppler/doppler.html

Drawing waves from moving source

Drawing waves…

If source is stationary

Source

( ( ( ( * ) ) ) ) )

←wave moves vsource = 0 wave moves→

If source is stationary

Source

( ( ( ( * ) ) ) ) )

←wave moves vsource = 0 wave moves→

If source moves

Source

( ( * ) ) ) ) )

Longer λ Shorter λ

Lower f Higher f

Red Shift Blue Shift

If source is stationary

Source

( ( ( ( * ) ) ) ) )

←wave moves vsource = 0 wave moves→

If source moves

Source

( ( * ) ) ) ) )

What if source moves faster?

( ( *)))))

Stretched more Compressed more

Higher speeds Bigger shifts

Other ex: Race cars, train

Same results if source or observer

or both move

Approach Shift to shorter λ Blue shift

Recede Shift to longer λ Red shift

Bigger shift in λ Bigger speed

Ex: “Earth’s Orbital Speed”

Ex: “Earth’s Orbital Speed”

Predict: If Earth is at A, will the star’s

spectrum be red-shifted or blue-shifted?

Ex: “Earth’s Orbital Speed”

Ex: “Earth’s Orbital Speed”

Spectral lines from source at rest

Ex: “Earth’s Orbital Speed”

What color are these lines?

Violet

Ex: “Earth’s Orbital Speed”

Where is the red end of the spectrum?

Ex: “Earth’s Orbital Speed”

Blue end of spectrum

Red end of spectrum

Ex: “Earth’s Orbital Speed”

Is “a” red shifted or blue shifted?

Ex: “Earth’s Orbital Speed”

Blue shift

Red shift

Ex: “Earth’s Orbital Speed”

Is “b” red shifted or blue shifted?

Ex: “Earth’s Orbital Speed”

Blue shift

Red shift

= 546 nm = 643 nm

Spectrum of element Xo (at rest) | | |

Spectrum of star A (at rest) | | | ||

From the spectra above, you can conclude that star A

1. Contains the element Xo and only that element

2. Contains the element Xo and at least one more element

3. Does not contain the element Xo

4. There is not enough information to determine the composition

= 546 nm = 643 nm

Spectrum of element Xo (at rest) | | |

Spectrum of star A | | | ||

From the spectra above, you can conclude that star A

1. Contains the element Xo and only that element

2. Contains the element Xo and at least one more element

3. Does not contain the element Xo

4. There is not enough information to determine the composition

From the spectra above, you can conclude that Earth and star A

1. Are moving toward each other

2. Are moving away from each other

3. There is not enough information to determine the relative direction of motion of Earth and the star.

= 546 nm = 643 nm

Spectrum of element Xo (at rest) | | |

Spectrum of star A | | | ||

Homework:

1. Quiz. Pick up at end of class or print it

from calendar.

Hand in on Thursday.

This is to learn from. OK to work with

each other but LEARN from it.

2. Tutorial will be emailed to you.

Not to hand in.

Ring of Truth – Doubt

Vera Rubin

http://nssdc.gsfc.nasa.gov/image/astro/hst_ngc4414_9925.jpg

Fritz Zwicky – 1930s

Galaxies in clusters are

moving too fast

Something holding them

together

Vera Rubin – 1960s

• Galaxies have bright centers

• Expect most mass at center

• Expect inner stars to move faster and outer stars to move slower.

• Like solar system

http://nssdc.gsfc.nasa.gov/image/astro/hst_ngc4414_9925.jpg

Vera Rubin – 1960s

• Instead, outer stars orbit about same speed as inner ones

• Lots of mass far from center

• 90% of mass is unseen

http://nssdc.gsfc.nasa.gov/image/astro/hst_ngc4414_9925.jpg

Gravitational Lensing

http://teacherlink.ed.usu.edu/tlnasa/pictures/litho/Abell2218/Abell2218.htm

Gravitational Lensing – 1980s

Bullet Cluster

2 clusters of galaxies moving apart after colliding

Red: X-rays (colliding gas)

Blue: dark matter (gravitational lensing)

http://antwrp.gsfc.nasa.gov/apod/ap080823.html

http://www.nasa.gov/mission_pages/hubble/news/dark_matter_ring_feature.html

Dark Matter

“Seen” by its gravitational pull

• Rotation of galaxies (Rubin)

• Motions of galaxies within clusters (Zwicky)

• Bending of distant galaxy light by

intervening clusters (Gravitational Lensing)

• Collision of two clusters of galaxies

Dark Matter

What is it?

MACHOs Neutrinos WIMPS

Sky & Telescope magazine

(Aug 2008 & Apr 2009)

Tower-Soudan mine