Transcript Monday, Sept. 22

Lecture 8 Phys 1810
Password!
Given out in Class, not email.
TODAY! Office hour
3pm Allen 514
To do practice questions for test/exam,
the textbook online code is required.
• Read BEFORE coming to class:
– Electromagnetic Radiation 3.1 to
3.4
• Energy
• Thermal Radiation Box 3-2
• Flux and Luminosity
(L equation in Box 17-2)
– Spectra 4.1, 4.2
• Kirkhhoff’s Laws
– Radio Emission 18.4
– Doppler shift: 3.5, Box 3-3, 4.5
– Telescopes 5.2, 5.3, “seeing”
in 5.4, 5.5-5.7
–
The class lecture website is
http://www.physics.umanitoba.ca/~english/2014fallphys1810/
[EM]
summary
Recall column
Electromagnetic Wave
• oscillations occurring perpendicular
to the direction of energy transfer
• oscillating electric & magnetic fields
 a B field accompanies a changing E field.
Thus a vibrating charged particle in a star create
EM waves in its own EM field and these waves
propagate through space.
Hole in wall
Hole in the Wall
Expected for particles
Observed
Check the class website for videos!
Double Slit  Interference Pattern
http://www.olympusmicro.com/primer/java/doubleslitwavefronts/index.html
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” ?
• https://www.youtube.com/watch?v=MbLzh1Y9POQ
Demonstrates the DUAL NATURE of light.
Particle Description  Photons
Photon Energy (E)
h== Planck constant.
but
So
Higher frequencies have higher energies
How does the speed of radio waves
compare to the speed of visible light?
They both travel at the same speed.
You may want to
write down what
I say about each
range and
transparency.
summary
Recall column
Thermal Radiation
• “heat”
• most familiar kind of radiation
• caused by random motions of atoms &
molecules
• a lot of energy available large amount
of motion (high temperature)
T== temperature
summary
Recall column
Blackbody Radiation
• blackbody (b.b.) radiation is thermal
radiation emitted a blackbody
• blackbody == “perfect absorber” &
re-emits radiation in all directions!
(doesn’t scatter)
• no “perfect” blackbody but close:
– some ovens
– stars (sun)
– cosmic background radiation
Temperature Scales
Recall column
summary
summary
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Blackbody Radiation
• b.b.s emit across a range of λ
• but intensity not the same at all λ
• Temperature (T) of b.b. determines
intensity of radiation & the peak λ
Blackbody Radiation
summary
Recall column
Intensity
Intensity
Wavelength 
b.b. radiation depends only on its T.
The intensity changes at different wavelengths.
(Graph of 1 object at 1 specific T.)
Blackbody Radiation: b.b. curve
summary
Recall column
Explained using
particle theory of light
Intensity
Intensity
photons of energy
Wavelength
Frequency 
b.b. radiation depends only on its T.
X-axis
Book uses increasing frequency (nu).
Most astronomers use increasing wavelength (lambda),
so we’ll use this.
Blackbody Radiation: Wien’s Law
Intensity
Wavelength 
Peak
Intensity
Recall column
summary
Blackbody Radiation Curves for Different Temperatures
Recall column
20,000° K
10,000° K
Intensity
5000° K
2000° K
1000° K
500° K
Wavelength (nm)
X-Ray
Ultraviolet
Visible Infrared
Microwave Radio
summary
Example using Wien’s Law:
.
E.g. If
is at short wavelengths for a b.b.,
then its T is higher & the object’s emission is
towards blue end of EM spectrum.
summary
Recall column
Star A
Star
B
Intensity
A plot of blackbody spectra
of five different stars is
shown in the figure. Based
on these spectra, the star
with lowest T is Star E.
Star
C
Star D
Star E
Wavelength
Short
Long
What colour does this star have?
Recall column
Red
summary
summary
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(greenish)
Yellow-white
summary
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Blue
what is hot & what is not?
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The hottest
stars in this
image appear:
a)Blueish
b)Reddish
summary
Contrast with everyday experience!
Recall column
summary
On supplemental page.
Examples of Blackbodies and their Temperatures.
summary
Recall column
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
Microwave/Infrare
d
Humans
310
9.7 μm
Infrared
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
Objects and Peak of Emission
Dense, spherical clouds:
radio and Far-IR
Globule of dust: IR
Sun: visible
White dwarf
star/planetary
nebula:
UV
Flux
- related to temperature (T)
Stefan-Boltzmann law for b.b:
Relates T to the total amount of energy (E) that
the b.b. emits at all wavelengths.
Balloon & Surface Area
• 2 stars with the same T but different surface
area.
• Total energy output over the whole sphere of
larger object is larger.
To here for the afternoon
Stars: Their Characteristics
summary
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• Luminosity (L): The total energy
radiated per second, at all wavelengths.
• L = surface area * flux
• Surface area of a sphere is

T== Surface
Temperature
Luminosity is proportional to the radius squared times surface temperature to the 4th power.
Stars: Why Temperature is useful.
summary
Recall column
• Notice that if we know the
temperature of a star, then if we
know the radius, we can calculate
the luminosity.
• Alternatively, if we know the
temperature and the luminosity we
can determine the radius.

summary
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The Interaction of light and matter.
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• Photons (γ == gamma)
– Individual packet of EM energy that makes up EM
radiation
• γ & matter interact creating spectra.
• Spectra used to assess
• T (blackbody curve type spectrum)
• processes that produce light or absorb it (i.e. what
is going on)
(Animation)
summary
summary
Spectra
Recall column
Kirchhoff’s Laws
• 3 empirical laws
a) Hot opaque body -> continuous spectrum
b) Cooler transparent gas between source & observer ->
absorption line spectrum
c) Diffuse, transparent gas -> emission line spectrum
Spectra
summary
Recall column
• This kind of spectrum (continuum) is caused
by
a) Hot, low density gas
b) Hot, dense blackbody
c) Cooler transparent gas
Spectra
summary
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• Our sun and other stars have an
atmosphere. Imagine that you are in a
spaceship far above the Earth’s
atmosphere. Which of the following
spectra would you observe when analyzing
sunlight?
a) Continuum rainbow-like spectrum
b) Dark line absorption spectrum
c) Bright line emission spectrum
summary
Spectral Finger Prints
Recall column
Solar Spectrum
• Note emission lines for lab spectrum of
iron are at same λs of absorption lines
of iron in 
• Can use line spectra to determine
chemical elements in object.
Interaction of Light and Matter:
Recall column
summary
How are line spectra created?
γs of light interact with atoms &
molecules.
• Atoms consist of:
– Electrons (negative charge) == e– Nuclei (balance charge of e-)
• Protons (positive)
• Neutrons (neutral)
• Molecules: group of 2 or more atoms.
Interaction of Light and Matter
Recall column
•
•
•
•
•
•
Hydrogen == H: simplest atom.
1 e- & 1 proton.
Classical picture: e- in an orbit.
Contemporary picture: e- as a cloud.
Orbits are really energy levels.
E == energy
summary
Interaction of Light and Matter
summary
Recall column
Hydrogen Atom Energy Levels
• Every chemical element has its own specific
set of E levels.
• Each E level is associated with a λ.
Interaction of Light and Matter
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summary
Creating spectral lines at visible wavelengths
• specific (quantized) E levels. Level with
lowest E is ground state.
• How does e- get excited?
– By interactions between γs & matter.
Interaction of Light and Matter:
Recall column
summary
Creating spectral lines at visible wavelengths
• The e- can shift between E levels by
absorption & emission of γs.
summary
Recall column
• To here for the morning.