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Physics We NeedTopic 1: Light and Doppler Shifts

Big Bang, Black Holes, No Math

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ASTR/PHYS 109Dr. David TobackLecture 8 & 9



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Prep For Today (is now due) – L9• Reading:

– BBBHNM Unit 2 (already due)• Pre-Lecture Reading Questions:

– If you were misgraded, need help or an extension let me know– Unit 1:

• Pass/Revise: You must get a 10/10• If you haven’t gotten a 10/10 submit a Revision. If you don’t pass

that, send revised questions to 109help– Unit 2: Will try to post grades soon– Unit 2 Revision: Will announce due date when we are done grading original

• End-of-Chapter Quizzes– (Chapter 4)

• Papers– Paper 0 (Reviewer Training):

• Nothing to write!• Reviews due before 11:55PM tonight• Back-Evaluations due before 11:55PM Wednesday• You are required to do all the parts of this assignment



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Outline for Light

1. Light is a wave…2.…and a particle3. Light is REALLY fast,

but does have a finite speed

4.Doppler effect



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PhotonsWhat do we know about

photons?• Photons are a type of

particle– Like an electron or proton

• Higher energy smaller wavelength

• Has no mass, but does have energy– Aside for those of you who are interested: It has momentum



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A Particle and a Wave

• Actually it’s really more complicated than that

• Light is both a wave and a particle… can describe it as either… it has properties of both



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Part 3: The Speed of Light

“Not just a good idea, it’s the law!”



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Light has a Finite Speed

Light has been measured to have a specific speed –3 x 108 meters per second –186,000 miles per second–1 foot per nanosecond–Really FAST–The fastest speed allowed



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Maxwell’s Theory• 1865: James Clerk Maxwell and others showed light was a wave and calculated its expected speed

• Gave the observed speed of light!– Confidence that his theories were correct

– We can now measure the speed of light to incredible precision Physics 2082.99792458 x 108 m/s



A Light-Year• Some things are so far away that it’s easier to express their distance in light-years–Light travels at 3 x 108

meters/second and there are about 3 x 107

seconds/year ~1016 m in a light-year

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A Time Machine-Like Viewer• The Sun is 8 light-minutes away, so we are seeing the Sun as it was 8 minutes AGO

• If a star is a light-year away and it must have taken a year for the light to get to us, we are observing the star the way it was a year ago

• If we look at light from something billions of light-years away, we are “observing” things that happened billions of years ago

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Part 4: The Doppler Effect• 1842: Christian Doppler

• What happens to waves when the “thing” creating the waves is moving towards us?–Or we’re moving towards it? Physics 221



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Example with Sound From a Stationary Car

The sound we hear is actually a wave–Sound waves



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Now the Car is MovingThings coming at us have their waves “scrunched up”

Things moving away have their waves “stretched”



The Doppler “Effect” for CarsHears a LowerPitched Sound

Hears a HigherPitched Sound

A car blaring its horn and going past you will give the famous “Eeeeeeee-Yowwwwww” sound 31



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What Does This Have to do with Light?

• Since both sound and light are waves, the Doppler “Effect” applies equally to both – Normally can’t see this effect for light since its wavelength is small and its speed is fast

• Can in special cases:– Things need to be moving really fast, like stars and galaxies

– Need really good measuring devices like the ones Astronomers have



Doppler Effect and LightBlue Shift Sees light

with a smaller

wavelength

Red Shift Sees light

with a larger

wavelength

Galaxy moves to the

right

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• The light from galaxies moving away from us will appear to have longer wavelength light– The light is more Red, or

shifted to the Red side of the light spectrum

– “Red Shifted”• The light from galaxies

moving towards us will appear with shorter wavelength– The light is more Blue, or

shifted to the Blue side of the light spectrum

– “Blue Shifted”

Look at Light from Galaxies



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The Doppler Effect in Action

Doppler Effect



Three ObserversWhat does an object emitting light look like to

different observers when it is 1) Not moving2) Moving

Doppler Shift

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Look at some casesNext lets look at two cases where an “explosion”

occurs and things move away from that location

1. I stay at the explosion point, but the other stuff goes off into space (I’m at the “center” of the explosion)

2. I’m just one of the thing that got blasted into space. I move along with them

1. How would this look to an outside observer?

2. How it would look to me in my reference frame?



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What it looks like when I’m stationary (or in my reference frame)

Case 1: Two Objects as they move away from an Explosion (I’m at the explosion site)



What it looks like when I’m moving relative to the

objects

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Case 2: Two Objects and Me Moving Away from the Explosion site (seen by someone else)



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Case 2: Two Objects and Me Moving Away from the Explosion Site (from my perspective)

Why were these cases important?Sorry… you’ll have to wait until we look at galaxies sending us light



Doppler Effect and Light: Summary

Compare the light coming from a car that’s parked and one that’s coming towards you

If a car is coming toward you with its lights on

1.The color of the light changes2.The speed of the light does

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Putting It All TogetherSince stars can be billions of light-years away, and are moving quickly, we can use the Doppler effect on light to measure the speed and direction of all objects in the night sky – More in Chapter 10(Can also use it to tell us what Stars are made of, but for that we need to learn about quantum mechanics…)



Lecture on Chapter 5 now complete

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Outline for Unit 2: Physics We Need

1. Light and Doppler Shifts Done

2. Gravity, General Relativity and Dark MatterNext

3. Atomic Physics and Quantum Mechanics

4. Nuclear Physics and Chemistry5. Temperature and Thermal

Equilibrium



Prep For Next Time – L9• Reading:

– BBBHNM Unit 2 (already due)• Pre-Lecture Reading Questions:

– If you were misgraded, need help or an extension let me know– Unit 1:

• Pass/Revise: You must get a 10/10• If you haven’t gotten a 10/10 submit a Revision. If you don’t pass

that, send revised questions to 109help– Unit 2: Will try to post grades soon– Unit 2 Revision: Will announce due date when we are done grading original

• End-of-Chapter Quizzes– If we finished Chapter 5 then End-of-Chapter Quizzes 5a & b (else just

Chapter 4)• Papers

– Paper 0 (Reviewer Training): • Nothing to write!• Reviews due before 11:55PM tonight• Back-Evaluations due before 11:55PM Wednesday• You are required to do all the parts of this assignment 47

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