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Physics We NeedTopic 1: Light and Doppler Shifts
Big Bang, Black Holes, No Math
1
Big Bang, Black Holes, No Math
ASTR/PHYS 109Dr. David Toback
Lecture 6 & 7
Physics We NeedTopic 1: Light and Doppler Shifts
Big Bang, Black Holes, No Math
2
Prep For Today (is now due) – L6
•Reading:– BBBHNM Unit 2 (Chapters 5-9)
•Pre-Lecture Reading Questions– Boot Camp Quizzes (already
due)– Unit 2: Stage 1 (due before
Class)– Unit 1 Revision (if needed): Will
send email after regrades finished
•End-of-Chapter Quizzes– Chapter 4
Physics We NeedTopic 1: Light and Doppler Shifts
Big Bang, Black Holes, No Math
3
Prep For Today (is now due) – L7
•Reading:–(BBBHNM Unit 2)
•Pre-Lecture Reading Questions–Unit 2: Stage 2 (due before Class)
–Unit 1 Revision (if needed): Will send email after regrades finished
•End-of-Chapter Quizzes–Chapter 4
Physics We NeedTopic 1: Light and Doppler Shifts
Big Bang, Black Holes, No Math
4
Next Set of Topics: Physics We Need
Finished our introduction•Big and small things in the Universe
•Evidence and the Scientific Method
Next we move on learn some of the physics we need in order to understand why scientists have confidence in the Big Bang Theory
Physics We NeedTopic 1: Light and Doppler Shifts
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Overview of Physics We Need
•Cosmology, the big bang and black holes are some of the most interesting things that can be understood
•However, to UNDERSTAND them better, and the EVIDENCE for them, we need to learn some physics
Physics We NeedTopic 1: Light and Doppler Shifts
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The Topics•Some stuff we need learn a little about:
1. Light and Doppler Shifts2. Gravity, General Relativity and Dark
Matter3. Atomic Physics and Quantum
Mechanics4. Nuclear Physics and Chemistry5. Temperature and Thermal
Equilibrium•We won’t spend too long on each, just enough to get back to the big picture…
•Since there is no perfect way to present them (they all tie into each other) we’ll just start somewhere and get going…
Physics We NeedTopic 1: Light and Doppler Shifts
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Back to the Big Picture
Questions we’re trying to answer:
•How did the Universe come into being?
•How did it evolve into what we know today?
•What is the evidence for all of this?
Physics We NeedTopic 1: Light and Doppler Shifts
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LightWhat we know about the universe comes from multiple places
One of the most important is from looking at both outer space and inner space
Need to know more about the “light we see”
Physics We NeedTopic 1: Light and Doppler Shifts
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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
Physics We NeedTopic 1: Light and Doppler Shifts
Big Bang, Black Holes, No Math
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Part 1: Light is a Wave
•The light we see is a “wave”, like waves on an ocean
•Best described by it’s wavelength
Physics We NeedTopic 1: Light and Doppler Shifts
Big Bang, Black Holes, No Math
Why do we believe light is a wave?
•Water and Sound are also both waves–Does light behave like water or sound?
•Can “see” water waves hitting a wall and what happens to them when they hit two small openings in a wall
Physics We NeedTopic 1: Light and Doppler Shifts
Big Bang, Black Holes, No Math
Water waves moving towards a wall
You are in the sky looking down at the waves as they go by
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Physics We NeedTopic 1: Light and Doppler Shifts
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Why do we believe light is a wave?
Shine light from a laser at two small holes in a wall and see what
comes out on the wall behind itThe waves make patterns on the
wall the same way water waves do (Physics 202 & 208)
Physics We NeedTopic 1: Light and Doppler Shifts
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The Wavelength of Light
•Shine white light at a prism and it breaks up into its different colors
•Each color has a different wavelength which we can measure
Physics We NeedTopic 1: Light and Doppler Shifts
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Light is more than the colors we see
You may have thought of Radio Waves and X-rays as different things, but they are just light with different wavelengths
Physics We NeedTopic 1: Light and Doppler Shifts
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Part 2: Light is a particle
•What if we look at smaller and smaller “amounts” of light?
•The light we see is actually LOTS of particles all moving together
•Call these Photons
Lots of photons where its
brightVery few photons where its
dark
Physics We NeedTopic 1: Light and Doppler Shifts
Big Bang, Black Holes, No Math
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White Light = Lots of Different Color Photons
Physics We NeedTopic 1: Light and Doppler Shifts
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Another view…
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Physics We NeedTopic 1: Light and Doppler Shifts
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The Energy of ParticlesWe can think about Energy in many
ways• One way is the “energy of
motion” – Kinetic Energy in the language
of PHYS 201 and 218• For a particle (with mass) in
motion:– The higher the speed the
particle has, the more energy it has
– The more mass the particle has (the heavier it is) the more energy it has
Physics We NeedTopic 1: Light and Doppler Shifts
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Clicker Question for Energy
Which has more energy? (Which will do more damage to a tree?)
1. A) A Prius moving at 10mi/hr or
B) An SUV at 10mi/hr?2. A) An SUV moving at 10mi/hr
or B) An SUV moving with
60mi/hr?3. A) A Prius moving at 60mi/hr
or B) An SUV moving at 10mi/hr?
C) Depends on the masses of the Prius and the SUV
Physics We NeedTopic 1: Light and Doppler Shifts
Big Bang, Black Holes, No Math
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Photons
What 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
Physics We NeedTopic 1: Light and Doppler Shifts
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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
Physics We NeedTopic 1: Light and Doppler Shifts
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Part 3: The Speed of Light
“Not just a good idea, it’s the law!”
Physics We NeedTopic 1: Light and Doppler Shifts
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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
Physics We NeedTopic 1: Light and Doppler Shifts
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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
Physics We NeedTopic 1: Light and Doppler Shifts
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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 x 107 seconds/year ~1016 m in a light-year
Physics We NeedTopic 1: Light and Doppler Shifts
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Time Machine• 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
Physics We NeedTopic 1: Light and Doppler Shifts
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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
Physics We NeedTopic 1: Light and Doppler Shifts
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Example with Sound From a Stationary Car
The sound we hear is actually a wave–Sound waves
Physics We NeedTopic 1: Light and Doppler Shifts
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Now the Car is Moving
Things coming at us have their waves “scrunched up”
Things moving away have their waves “stretched”
Physics We NeedTopic 1: Light and Doppler Shifts
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The Doppler “Effect” for Cars
Hears a LowerPitched Sound
Hears a Higher
Pitched Sound
A car blaring its horn and going past you will give the famous “Eeeeeeee-Yowwwwww” sound
Physics We NeedTopic 1: Light and Doppler Shifts
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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
Physics We NeedTopic 1: Light and Doppler Shifts
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Doppler Effect and Light
Blue Shift Sees light
with a smallerwavelen
gth
Red Shift Sees light
with a larger
wavelength
Galaxy moves to the right
Physics We NeedTopic 1: Light and Doppler Shifts
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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
Physics We NeedTopic 1: Light and Doppler Shifts
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The Doppler Effect in Action
Doppler Effect
Physics We NeedTopic 1: Light and Doppler Shifts
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Three ObserversWhat does an object emitting light look
like to different observers when it is 1)Not moving2)Moving
Doppler Shift
Physics We NeedTopic 1: Light and Doppler Shifts
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Look at some cases
Next lets look at three cases where an “explosion” occurs and things expand from that point
1. I’m stationary and the exploding things are moving away from me (I’m the center of the explosion)
2. I’m moving along with the exploding things
3. Same as case 2, but how it would look to me in my reference frame
Physics We NeedTopic 1: Light and Doppler Shifts
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Two Objects Moving Away from Me
What it looks like when I’m stationary (or in my reference
frame)
Physics We NeedTopic 1: Light and Doppler Shifts
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Two Objects Moving Away From Me
What it looks like when I’m moving relative to
the objects
Physics We NeedTopic 1: Light and Doppler Shifts
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Same Thing: My Reference Frame
Why were these three cases important?
Sorry… you’ll have to wait until we look at galaxies
sending us light
Physics We NeedTopic 1: Light and Doppler Shifts
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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 changes
2.The speed of the light does not!
Physics We NeedTopic 1: Light and Doppler Shifts
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Putting It All Together
Since 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(Can also use it to tell us what Stars are made of, but for that we need to learn about quantum mechanics…)
Physics We NeedTopic 1: Light and Doppler Shifts
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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 Chemistry
5. Temperature and Thermal Equilibrium
Physics We NeedTopic 1: Light and Doppler Shifts
Big Bang, Black Holes, No Math
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Prep For Next Time – L6
•Reading:– (BBBHNM Unit 2)
•Pre-Lecture Reading Questions– Boot Camp Quizzes (already
due)– Unit 2: Stage 2 (due before
Class)– Unit 1 Revision (if needed): Will
send email after regrades finished
•End-of-Chapter Quizzes– If we finished Chapter 5 then
End-of-Chapter Quiz 5 (else just Chapter 4)
Physics We NeedTopic 1: Light and Doppler Shifts
Big Bang, Black Holes, No Math
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Prep For Next Time – L7• Reading:
– (BBBHNM Unit 2)• Pre-Lecture Reading Questions
– Unit 2 Revision: Stage 1 (due before Class)
– Unit 1 Revision (if needed): Will send email after regrades finished
• End-of-Chapter Quizzes– If we finished Chapter 5 then End-of-
Chapter Quiz 5 (else just Chapter 4)
Physics We NeedTopic 1: Light and Doppler Shifts
Big Bang, Black Holes, No Math
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Physics We NeedTopic 1: Light and Doppler Shifts
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Clicker QuizQ: What does it mean for an electron to
be "stable?"a)It is balanced with exactly one protonb)Its orbit ring exists in one plane (no
wobble)c)It has reached minimum size (10-19 m) d)Undisturbed, it can go on living
forever and evere)They don't appear to be made of
anything smaller
Physics We NeedTopic 1: Light and Doppler Shifts
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Clicker Quiz
Which of the following travels the fastest?
A.Radio wavesB.Gamma raysC.Red lightD.Blue lightE.They all move
at the same speed
Physics We NeedTopic 1: Light and Doppler Shifts
Big Bang, Black Holes, No Math
Clicker QuizWe observe two objects in
space. From other studies, we can tell that object X is emitting infrared light and the object Y is emitting Ultraviolet light. We observe the color of the light object X to be Red and the color of Object Y to be blue.
Which statement is true?a) X and Y are moving towards
usb) X and Y are moving away
from usc) X is moving towards us and Y
is moving away from usd) X is moving away from us and
Y is moving towards us
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Physics We NeedTopic 1: Light and Doppler Shifts
Big Bang, Black Holes, No Math
Paper 0, Questions 1-51. Does the paper make it easy to understand what question is being answered, what the
answer is, and how it will be answered? If anyone of those are missing, or if you can't follow the argument, then choose No. If the argument is clear, but not said very well (or is wordy), then you should still give a Yes. If the paper is clear to you, but would not be clear to someone who hasn’t taken the course choose No. Being easy to read isn't a good enough reason to give a Yes. Having a sentence or two that are hard to understand (or poorly written) within a basically clear paper is fine, and can still deserve a Yes if it is otherwise ok.
2. Are the pieces of evidence clearly and correctly enumerated in the opening paragraph? This includes the electrons going around atoms, the Moon orbiting the Earth, and planets orbiting the Sun.
3. Does the structure of the paper follow the instructions? Specifically: Is there an introduction paragraph, separate paragraphs discussing evidence, and a conclusion paragraph? If any are missing, choose No. Does the paper directly address the question and make the argument to answer it? If the paper addresses the question and makes an argument (right or wrong) to answer it, choose Yes (we will address the wrongness in a later question). Do not choose No due to a poorly written or confusing introduction. The paper does not need to have only three paragraphs, and the pieces of evidence can be intertwined within them. Do not choose No due to missing evidence or more than three paragraphs. Do not choose No just because evidence is not constrained to these paragraphs. If the paper does not end with a conclusion paragraph, choose No. Do not choose No due to a poorly written or confusing conclusion.
4. Is the paper generally free of extraneous evidence? If there is any significant evidence that is off topic or an argument that has nothing to do with the topic, choose “No.” If there is significant evidence from outside class, choose No. Do not choose “No” due to evidence that is unnecessary if the evidence is relevant to the topic.
5. Does the paper only use language and words that can be understood by the average lay-person? If the author uses an acronym, like GR, for general relativity that is ok but only if the acronym is clearly defined. If they use an acronym that isn't defined, then select No. Similarly, just because you saw these words in a high-school science class does not mean they are understood by the average person. The average person on the street would know what an atom is, but not know what a spectral line is.
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Physics We NeedTopic 1: Light and Doppler Shifts
Big Bang, Black Holes, No Math
Paper 0, Questions 6-106. Does the essay use language formal enough to give to a
statesman? If the paper uses jokes, slang or other informal language, choose No. Do not choose No due to poor grammar, spelling mistakes, incomplete sentences, or poor sentence structure (we will take off points in a different question for this). Do not choose No due to the use of an acronym, like USA, since it is common usage and not slang.
7. Is the essay generally free of instances of bad grammar, incomplete sentences or spelling mistakes? If there are more than two minor mistakes, choose No.
8. Does the paper make clear how and why the evidence presented relates to the argument? If the facts are mentioned but show no clear connection to the argument, then choose No. If the facts are not addressed throughout the paper, then choose No. If it states the data without saying why it is evidence then choose No.
9. Do the middle (evidence) paragraphs clearly tie into the overall structure?
10.Does the paper use the scientific method and evidence based decision making? It’s OK if the words “scientific method” and “evidence-based decision making” are not mentioned specifically.
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Physics We NeedTopic 1: Light and Doppler Shifts
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Paper 0, Questions 11-1511.Does the essay make it clear what the Scientific
Method is and why it is important in this paper?12.Does the essay clearly tie together the pieces of
evidence using the scientific method in the conclusion?13.Does this paper say that the Moon orbits the Earth?14.Does the paper mention explicitly that the Earth is
much more massive than the Moon, and that it is important to why the Moon orbits the Earth instead of the other way around? If it mentions that the Earth is more massive, and that this is what is important, then this is good enough. If it just says that it is more massive without saying why that is important in some way, then answer No. If it says "bigger" or "larger" (or that the Moon is "smaller") that is good enough.
15.Does the paper mention that the Earth or the planets orbit the Sun? Either is fine. If neither is mentioned, then give a No. 52
Physics We NeedTopic 1: Light and Doppler Shifts
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Paper 0, Questions 16-2016.Does the paper mention explicitly that the Sun is much more massive
than the things orbiting it, and that it is important to say why they orbit the Sun instead of the other way around? If it mentions that the Sun is more massive, and that this is what is important, then this is good enough. If it just says that it is more massive without saying why that is important in some way, then answer No. If it says "bigger" or "larger" (or that the planets are "smaller") that is good enough.
17.Does the paper mention that electrons orbit protons or that electrons orbit a nucleus? Either one is fine.
18.Does the paper mention explicitly that the proton (or nucleus) is much more massive than the electron orbiting it, and that it is important to why they orbit the proton (or nucleus) instead of the other way around? If it mentions that the proton or nucleus is more massive, and that this is what is important, then this is good enough. If it just says that it is more massive without saying why that is important in some way, then answer No. If it says "bigger" or "larger" (or that the electron is "smaller") that is good enough.
19.Does the essay spend an appropriate amount of time on each topic?20.Does the conclusion paragraph tie together all the big picture issues
discussed in the other paragraphs? If the conclusion does not summarize the evidence, then chose No. If the conclusion simply re-states the pieces of evidence, then choose No.
53
Physics We NeedTopic 1: Light and Doppler Shifts
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Prep For Next Time – L7
•Reading:– (BBBHNM Unit 2)
•Pre-Lecture Reading Questions– eCampus quizzes (all 6)– Unit 1 Revision (if needed):
Stage 1 •Let us know if you were
misgraded– Unit 2: Stage 2
•End-of-Chapter Quizzes– If we finished Chapter 5 then
End-of-Chapter Quiz 5 (else just Chapter 4)
Physics We NeedTopic 1: Light and Doppler Shifts
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Prep For Today (is now due) – L7
• Reading: – BBBHNM Unit 2
• Pre-Lecture Reading Questions– eCampus quizzes (all 6)– Unit 1 Revision (if needed):
Stage 1 due date postponed until Monday•Let us know if you were misgraded
– Unit 2: Stage 1• End-of-Chapter Quizzes
– Chapter 4
Physics We NeedTopic 1: Light and Doppler Shifts
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Prep For Today (is now due) – L8
• Reading: – (BBBHNM Unit 2)
• Pre-Lecture Reading Questions– eCampus quizzes (all 6)
•Many of you need to check these
– Unit 1 Revision (if needed): Stage 1•Let us know if you were misgraded
– Unit 2: Stage 2• End-of-Chapter Quizzes
– (Chapter 4)
Physics We NeedTopic 1: Light and Doppler Shifts
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Prep For Next Time – L6• Reading:
– Required: BBBHNM Unit 2– Recommended Reading:
• See P3 of http://people.physics.tamu.edu/toback/109/Syllabus.pdf
• Pre-Lecture Reading Questions– Unit 1 Revision (if Needed): Stage 2 in CPR
due Monday before class– Unit 2: Stage 2 due Monday before class
• End-of-Chapter Quizzes– If we finished Chapter 5 then End-of-
Chapter Quiz 5 (else, just through Chapter 4)
Physics We NeedTopic 1: Light and Doppler Shifts
Big Bang, Black Holes, No Math
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Prep For Next Time – L7• Reading:
– Required: BBBHNM Unit 2– Recommended Reading:
• See P3 of http://people.physics.tamu.edu/toback/109/Syllabus.pdf
• Pre-Lecture Reading Questions– Unit 1 Revision: Let me know if you were
misgraded– Unit 2: Let me know if you were misgraded– Unit 2 Revision (if desired): Stage 1 due
Monday before class• End-of-Chapter Quizzes
– If we finished Chapter 5 then End-of-Chapter Quiz 5 (else, just through Chapter 4)
Physics We NeedTopic 1: Light and Doppler Shifts
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Was due today – L7• Reading:
– BBBHNM: All reading through Chapter 6
• Pre-Lecture Reading Questions:– All reading questions through
Chapter 6• End-of-Chapter Quizzes:
– All end-of-chapter quizzes through 4• Papers:
– Should have been working on Paper 0
– Read the “Writing Prompt” and other stuff on CPR
Physics We NeedTopic 1: Light and Doppler Shifts
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Paper 1 Stuff• I have extended the due date until
Friday at 5:45PM• Want feedback before turning it in?
– Post it in eLearning in .doc (or .pdf) for comments by the TA by 5PM tomorrow
– Make sure you polish it•If you have lots of grammar and spelling mistakes we’ll just make you resubmit it
– Will be due in CPR and turnitin.com in one week 60
Physics We NeedTopic 1: Light and Doppler Shifts
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Prep for Next Time – L6
• Reading: – BBBHNM 6
•Recommended: BHOT 4-6• Reading questions
– Two questions from Chapter 6 or the recommended reading
• End-of-Chapter Quizzes– If we finished Chapter 5 then
end-of-chapter quiz 5 (else just 4)
• Be working on Paper 1
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Prep for Next Time – L7
• Reading: – None
• Reading questions– None
• End-of-Chapter Quizzes– If we finished Chapter 5 then
end-of-chapter quiz 5 (else just 4)
• Paper 1– Phase II is due Friday at 5:45PM
Physics We NeedTopic 1: Light and Doppler Shifts
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Prep For Next Time – L6• Note: May change depending on how far we
get in lecture• Reading
– BBBHNM: None• Reading Questions:
– None• End-of-Chapter Quizzes
– If we finished Chapter 5 then end-of-chapter quiz 5 (else just up to Chapter 4)
• Paper Stuff– Paper 0 in CPR and turnitin.com before
class– Will talk more about Calibrations next time
Physics We NeedTopic 1: Light and Doppler Shifts
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Prep For Next Time – L7• Note: May change depending on how far we
get in lecture• Reading
– BBBHNM: None• Reading Questions:
– None• End-of-Chapter Quizzes
– If we finished Chapter 5 then end-of-chapter quiz 5 (else just up to Chapter 4)
• Paper Stuff– Paper 0 in CPR and turnitin.com before
class– Will talk more about Calibrations next time
Physics We NeedTopic 1: Light and Doppler Shifts
Big Bang, Black Holes, No Math
Full set of Readings So Far
•Required:–BBBHNM: Chap 1-6
•Recommended:–BHOT: Chap. 1-6–SHU: Chap. 1-3–TOE: Chap. 1
65
Physics We NeedTopic 1: Light and Doppler Shifts
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Prep for Today (is due now) – L6
•Reading:–BBBHNM: Chap 5
•Reading Questions:–Two questions from Chapter 5 or the recommended reading
•End-of-Chapter Quizzes–Chapter 4
Physics We NeedTopic 1: Light and Doppler Shifts
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Prep for Today (is due now) – L7
•Reading:–BBBHNM: Chap 6
•Reading Questions:–Two questions from Chapter 6 or the recommended reading
•End-of-Chapter Quizzes–Chapter 4
•Paper 1–Phase I (CPR and turnitin) was due Friday
–Phase II (Calibrations) due Friday at 5:45
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Why do we believe light is a wave?
• How do waves behave?
• Water waves and Sound waves
• Can “see” water waves hitting a wall and what happens to them when they hit two small openings in a wall
Big Waves Here
Small Waves Here
Waves move this way
Physics We NeedTopic 1: Light and Doppler Shifts
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Phase I, Phase II and Phase IIIPhase I: (Paper due 1 week after we finish
Chapter 4)• Turn in a polished draft to eLearning• TA’s will provide feedback, but not if there are
multiple spelling/grammar mistakes• Pass/Fail gradePhase II: (Due one week after Phase I)• A revised version will be done 1 week later on
CPR• When you submit to CPR (after revision), you
will need to also submit a copy of your to the turnitin.com line on eLearning
Phase III: (Due one week after Phase II)• Calibrations and Reviewing due one week after
you submit
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eLearning Quizzes – L8
•You are expected to be working on the quizzes on eLearning
•You cannot pass this course without having gotten a perfect score on all of them
•You make take as many of each quiz as you like to get perfect scores without penalty
•Due before the class AFTER we finish a chapter
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In-Class Quizzes
•Turn in your two questions on the reading
•During class you will write down two more–This is a separate quiz
• If you didn’t get a grade you wanted on one of the first few reading quizzes see me about trying again
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What if we look more closely?
•What if we look at smaller and smaller “amounts” of light?
•The light we see is actually LOTS of particles all moving together
•Call these Photons
LOTS of photons
Lots of photonsA few photons
Physics We NeedTopic 1: Light and Doppler Shifts
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In-Class Writing Assignment
Write down, during the course of lecture,
Two questions you want to know the
answer to
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Reading
Full reading so far:• BBBHNM: Chap 1-6• Prelude • BHOT: Chap. 1-6, 9 and 11
(117-122)• SHU: Chap. 1-3, 6, 7 (up-to-
page 153)• TOE: Chap. 1
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Before we start the next topic
•Don’t forget to take your eLearning quizzes
– Can’t pass the course without getting 100% on all of them
•Make sure your name and DATE are on EVERYTHING you turn in
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How do we answer these questions?
As previously discussed we have two avenues:
•Theory•ExperimentWhat do both have to say about Light?
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Light as a Wave
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Quiz
•What time is lecture on Wednesday?
•Where are we meeting?
–Name–Today’s date
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Before we begin
• As a special treat the class will attend a special Department of Physics colloquium on the LHC this Wednesday (Sept 17th)
• Class will start at 4PM in the teaching wing of the Engineering/Physics building, room 202 (big class room)
• Class will be over by 5PM• You will be expected to sign in
with me or Mike as your in class quiz
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Light is more than the colors we see
X-raysRadio WavesInfrared
Visiblecolor
Ultraviolet Microwaves
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In-Class Writing Assignment
Write down, during the course of lecture
Two questions you want to know the
answer to
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WebCT
• It has come to my attention that people are not finishing the quizzes in WebCT
• If they don’t get finished we will start requiring people to do them in Recitation to help enforce that they are done– You should be done with Quizzes
1-3 now– Done with Quizzes 4-6 by the
end of next week
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A Different Example
It is as if a traveling salesman were to send a letter home once a week during his travels
• While he is traveling away from home, each successive letter will have a little farther to go than the one the week before Letters will arrive a little more than a week apart
• On the way home each successive letter will have a shortest distance to travel will arrive more frequently than once a week
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Old beliefs•Light is so fast that people used to believe that it was simply instantaneous
•You can measure the speed of sound pretty easily–Listen to the alumni side at a football game. When they sing, they’re always LATE!
•Couldn’t answer the question with an experiment until 1600’s
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Why do we believe it has a finite speed?
• Danish Astronomer Ole Christian Roemer in 1676
• Moons of Jupiter are helpful again!
• Look at the eclipses of the moons of Jupiter as it moves towards us and away from us
We can measure this really
accurately now, but that’s another
story…
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Eclipses of the Jupiter’s MoonNow you see
it!
Now you don’t!
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Measure the Time Between Eclipses
•When Jupiter is moving towards us we get shorter times between eclipses
•When Jupiter is moving away from us we get longer times between eclipses
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Announcements
•Recitation tomorrow in ENPH 210
•No office hours today–Call/email if you need to meet
•WebCT Quizzez
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Light: Wave and Particle
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•Stars can be billions of light years away!
•Binary system: SHU p68•Had a way of measuring both the direction and speed of travel of every source of light in the night sky
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• However, in reality they are separate particles (feynman diagram?) and each photon interacts with an atom individually. Again a diagram?
•Individual interacting with an atom (Figure here)
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•Question: How do we measure the distance to Jupiter?
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Doppler Effect and Light
Replace with P65 from SHU
• Galaxies moving towards us have shorter wavelength light•The light is more Blue, or
shifted to the Blue side of the light spectrum
•“Blue Shifted”• Galaxies moving away from
us have longer wavelength light•The light is more Red, or
shifted to the Red side of the light spectrum
•“Red Shifted”
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•We know the Doppler effect in sound… Both volume and pitch change. Picture SHU p65
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Why do we believe light is a wave?
•Shine white light at a prism and it breaks up into its different colors
•These different colors are waves with different wavelengths
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Light is a wave
The light we see is a “wave” and it is best described by it’s wavelength
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•This is a test
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