1. Perspective on the Universe Phys 1830 Lecture 7
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Previous Class:
Order of Magnitude example
Distances:
parallax out to 200 pc
parsec
Inverse Square Brightness Law for distant stars
This Class
electromagnetic radiation
Next Class
the interaction between matter and light
Black & White workshop coming up!
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Considered the vastness of the universe – converting kpc to ly we found that a signal from MW centre takes at least 24,000 years! Hard for civilizations to communicate.
Change of password
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Dr. English your name, iclicker # & student # AND course.

2. Material tested is from quiz to quiz.
Quizzes are based on material from the previous Friday and the current Monday and Wednesday.

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4. Gravity! Light behaving according to the General Theory of Relativity.
What are we seeing here?
We even use light to study gravity.
Arcs are distortions caused by a foreground gravitational lens consisting of a cluster of galaxies.
Abell Cluster 2188 imaged by HST.
Gravity! Light behaving according to the General Theory of Relativity.

5. Our Universe
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Abell 1689
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Galaxy Cluster and Lensed Galaxy
2.2 billion light-years (to lensing cluster)
12.8 billion light-years (to lensed galaxy)
information about our universe comes from light
electromagnetic radiation
very distant objects (basically anything beyond our Solar System) are inaccessible for direct study
all the information we have about these objects comes from their radiation

6. The Electromagnetic Spectrum
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(EM)
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Electromagnetic radiation spans a range larger than what our eye detects. The arrow indicates the direction wavelength increases and energy decreases. Regions of the spectrum are categorized as Gamma Rays (high energy), X-rays, visible/optical light, Infra-red radiation, Microwaves and Radio waves.

7. the wave propagates through space at the speed of light = 300 000 km/s
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Imagine placing your hand in a pond of water and creating ripples. These are waves with the features in the diagram.
EM Radiation is a wave
the wave propagates through space at the speed of light = km/s
all EM Radiation: Radio through Visible through X-Rays propagates at the same speed!

8. Frequency & Wavelength
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The speeds of both waves are the same
The upper wave has a shorter wavelength and a higher frequency
The lower wave has a longer wavelength and a lower frequency
High frequency: more waves pass per second!
This wavelength is twice as long

9. The Electromagnetic Wave
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demos.
changing electric field generates a magnetic field
changing magnetic field generates an electric field
Demos
- take notes

10. Electromagnetic Wave
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See another good animation:
oscillations occurring perpendicular to the direction of energy transfer
oscillating electric & magnetic fields

11. Velocity of a Wave velocity = wavelength (λ) * frequency (ν)
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velocity = wavelength (λ) * frequency (ν)
Velocity of an Electromagnetic Wave?
Speed of light! Abbreviated: c
c = λν
c = 3 * 108 m/s (= 3 * 10 km/s)
λ has units of nanometers (10-9 m) or sometimes Ångströms = Å = m
ν has units of Hertz = s-1 (or per second) 5
wavelength – lambda
frequency – nu
Recall that “*” = “times” (multiplication)

12. Wavelength & Frequency
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because the speed of light is a constant (c = 3 x 108 m/s) if we know the wavelength, we can calculate the frequency!
What is the frequency of 550 nm green light? (re-arrange equation)
ν = c / λ
ν = 3 * 108 m/s / 550 x 10-9 m
ν ~ 5 * 1014 /s
ν ~ 5 * 1014 Hz
What frequencies do radios operate at? How do they compare with visible light frequencies?

13. iClicker Question
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How does the speed of radio waves compare to the speed of visible light?
Radio waves are much slower.
They both travel at the same speed.
Radio waves are much faster.

14. nanometer = nm = 10-9 m Ångströms = Å = 10-10 m
Visible Light
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nanometer = nm = 10-9 m Ångströms = Å = m
phenomenon known as dispersion
pass ordinary sunlight through a prism and you will see a rainbow
prism refracts (bends) the light
blue light refracts more than red light, so the colours separate out

15. In nature - the wondrous rainbow
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Drops of water suspended in the air.

16. The Nature of Light Two Theories Historically Waves (Huygens)
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Two Theories Historically
Waves (Huygens)
Particles (Newton)
both used in modern times
Wave Theory Phenomena
diffraction
interference
polarization
Particle Theory Phenomena
photons
spectral lines
black bodies
Light is both a wave and a particle – it has a DUAL nature.
diffraction – how light spreads out
interference – how waves add together to form a new wave pattern
polarization – how waves are oriented ... e.g. oriented so that they are travelling in 1 direction via polaroid glasses
photons – packets of energy ... One can think of photons as packets of waves.
spectral lines – e.g. emission from one element, like neon
black bodies – emit photons at all wavelengths... see later in this lecture

17. Theoretical Predictions
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side on view
face on view
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if round hole in wall
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e.g. “baseballs”
e.g. “water waves”

18. What happens if you send photons one at a time through a double slit?
Double Slit  Interference Pattern
You can do a version of the double slit at home using a laser pointer and covering a small comb with tape, except for 2 slits.
Some light sources emit 1 photon at a time, so this experiment has been done.
Light is both a wave and a particle !
What happens if you send photons one at a time through a double slit?
Would you get only 2 strips as if the photons were “baseballs” ?
Demonstrates the DUAL NATURE of light.

19. Thermal Radiation
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this is the most familiar kind of radiation (there are other kinds too!)
thermal radiation is caused by the random motions of atoms and the electrons in the atoms
if there is a lot of energy available, there will be a large amount of motion (indicated by a high temperature)
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20. Thermal Radiation high thermal energy produces high energy photons!
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high thermal energy produces high energy photons!
high energy photons = Gamma Rays, X- Rays,
low energy photons = Radio waves
thermal motion is random motion
motions can also be due to magnetic fields or nuclear processes - this radiation is not thermal
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21. Temperature Scales We use the Kelvin scale for astronomical phenomena.
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Zero points are all different. All objects above absolute zero have some random (thermal) motions! At absolute zero, all random motions stop!
We use the Kelvin scale for astronomical phenomena.

22. Blackbody Radiation
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blackbody radiation is thermal radiation emitted by an object (a blackbody)
a blackbody is an object that is a “perfect absorber” (it absorbs all the radiation that hits it) - it then re-emits that radiation in all directions!
there is no “perfect” blackbody in practice, but the Sun (and all other stars) come close! Even better is the Cosmic Microwave Background Radiation.
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23. Blackbody Radiation
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all objects are blackbodies to some degree (everything will absorb some radiation, and everything re-emits this radiation with varying degrees of efficiency)
blackbodies don’t just emit at one wavelength, they always emit across a range of wavelengths
but, the intensity of the radiation will not be the same at all wavelengths
the temperature of the blackbody determines the intensity of the radiation, and the peak wavelength
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24. Blackbody Radiation Explained using particle theory of light
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Explained using particle theory of light
photons of energy E=hν
Intensity
Got to here:
Wavelength increases to the right; frequency increases to the left. High frequency = high energy = bluer colour. Low frequency = low energy = redder colours.
E == energy
h == is a constant
nu == frequency
Wavelength 
The radiation emitted by a blackbody depends only on its temperature.
This graph shows how the intensity of the radiation changes at different wavelengths.
This graph is for an object at one specific temperature.

25. Blackbody Radiation Curves for Different Temperatures
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Wavelength (nm)
500° K
1000° K
2000° K
5000° K
10,000° K
20,000° K
X-Ray Ultraviolet Visible Infrared Microwave Radio
Intensity
Each curve is called a spectrum, the plural is called spectra. A spectrum shows how intensity of radiation changes with wavelength.
The position of the peak shifts towards shorter wavelengths (higher energy) as the temperature increases. Intensity increases dramatically as the temperature increases.

26. Thermal Radiation from Astronomical Objects
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Examples of Blackbodies and their Temperatures.
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Thermal Radiation from Astronomical Objects
Object
Temperature (K)
Peak Wavelength
Electromagnetic Region
Cosmic Background 3
1 mm
Microwave
Molecular Cloud (stellar cores) 10
300 μm
Infrared
Humans
310
9.7 μm
Incandescent Light Bulb
3000
1 μm or 10,000 Å
Infrared/Visible
Sun
6000
5000 Å
Visible
Hot Star
30,000
1000 Å
Ultraviolet
Intra-Cluster Gas
100,000,000
0.3 Å
X-Ray
Cosmic Microwave Background radiation is the best physical example of a blackbody.
Intra-cluster gas is the gas between galaxies within a cluster of galaxies.

27. Match these objects to peak temperatures in the previous table.
Objects and Peak of Emission
Dense, spherical clouds:
radio and Far-IR.
Exercise for at home:
Match these objects to peak temperatures in the previous table.
Globule of dust:
IR.
IR spitzer and UV galex.
Sun:
visible
White dwarf star/planetary nebula: UV

28. Star A Star B Star C Star D Star E
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Intensity
Short Long
Wavelength
Star A
Star B
Star C
Star D
Star E
A plot of the blackbody spectra of five different stars is shown in the figure. Based on these spectra, which of the stars has the lowest temperature?
Star A
Star B
Star C
Star D
Star E

29. What colour does this star have?
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Red
The colour stripes give you an idea of the visual range for electromagnetic radiation. The curve represents the spectrum of the star.

30. summary
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Yellow

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Blue

32. what is hot & what is not? The hottest stars in this image appear:
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The hottest stars in this image appear:
Blueish
Reddish
Relate this to public outreach images.

33. Contrast with everyday experience!
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Note that the public audience reads “blue” as cold. Artists call blues “cool” and yellow + reds “warm”.

34. The Interaction of light and matter.
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Photons and matter interact creating spectra.
spectra can be used to assess
temperature (blackbody curve type spectrum)
processes that produce light or absorb it (i.e. what is going on)

35. Kirchhoff’s Laws Spectra 3 empirical laws
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 means “gives”
Astronomical objects have spectral “finger prints”.
Also called “Continuum emission” can be produced by a hot dense gas. (Like a light bulb – a rainbow of colours.) If we plot this as intensity versus wavelength this produces a blackbody curve.
c) Emission lines are produced by diffuse (low density) gas. (Like a neon sign.)
b) Place the diffuse gas in front of the hot, gas and absorption-lines are created. There is the continuum (rainbow) with dark absorption lines where the emission lines would be.
(show animation)
Kirchhoff’s Laws
3 empirical laws
Hot opaque body -> continuous spectrum
Cooler transparent gas between source & observer -> absorption line spectrum
Diffuse, transparent gas -> emission line spectrum

36. Next class: Kirchhoff’s Laws Spectra
Phys 1830: Lecture 7
Next class:
Kirchhoff’s Laws
Spectra
How the interaction of light and matter produce spectra.