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The Interaction of Light and Matter
• Spectral Lines
• Photons
• Rutherford-Bohr Model of the
Atom
• Quantum Mechanics and
Wave-Particle Duality
How can we know about the stars…?
• French Philosopher Auguste
Comte (1798-1857) stated
that it would not be possible to
know the chemical or
mineralogical nature of the
stars…
•Knowledge of the stars and other
astrophysical objects relies on the
information that is provided to us
through electromagnetic radiation….
•To do astronomy one must know
physics…Need to understand light and
how it interacts with matter. Fortunately
we understand a lot…
Fraunhofer Spectral Lines
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Josef von Fraunhofer(17871826) had catalogued 475 dark
spectral lines in the solar
spectrum.
Fraunhofer showed that we can
learn the chemical composition
of the stars. Identified a spectral
line of sodium in the spectrum of
the Sun
Spectral Lines
Kirchoff’s Laws of Spectra
•A hot dense gas or hot solid
object
produces
a
continuous spectrum with
no dark spectral lines
•A hot, diffuse gas produces
bright
spectral
lines
(emission lines)
•A cool, diffuse gas in front
of a source of a continuous
spectrum
produces
dark
spectral lines (absorption
lines) in the continuous
spectrum.
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Robert Bunsen (1811-1899) created a
burner that produced a “colorless” flame
ideally suited for studying the spectra of
heated substances.
Gustav Kirchoff (1824-1887) and Bunsen
designed a spectroscope that could
analyze the emitted light.
Kirchoff determined that 70 of the dark
lines in the solar spectrum corresponded
to the 70 bright lines emitted by iron vapor.
Chemical Analysis by Spectral
Observations “Spectral Fingerprint”.
Spectral Lines
(1868) New element discovered in Sun!!
• Helium discovered in
solar spectrum by Pierre
Janssen in 1868
Spectral Lines
Application of Spectral Measurements
• Stellar Doppler Shift
• Galactic Doppler Shifts
• Quasar Doppler Shifts
Spectral Lines
Spectrographs
• Spectroscopy
• Diffraction grating equation
dsinq = nl
(n=0,1,2,…)
• Resolving Power
Photons
Photoelectric Effect
• Photoelectric Effect
• Kinetic energy of ejected
electrons does not depend on
intensity of light!
• Increasing intensity will
produce more ejected
electrons.
• Maximum kinetic energy of
ejected electrons depends on
frequency of light.
• Frequency must exceed
cutoff frequency before any
electrons are ejected
Einstein took Planck ’ s assumption of
quantized energy of EM waves seriously.
Light consisted of massless photons
whose energy was:
E photon = hn =
hc
l
K max = E photon - F = hn - F =
hc
l
-F
Einstein was awarded the Nobel Prize in
1921 for his work on the photo-electric
effect
Photons
Compton Scattering
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Compton Scattering
Wikipedia entry
Arthur Holly Compton (18921962) provided convincing
evidence that light manifests
particle-like properties in its
interaction with matter by
considering how (x-ray) photons
can “collide” with a free electron
at rest.
Conservation of energy and
momentum leads to the
following:
Showed that photons are
massless yet carry momentum!!!
E photon = hn =
hc
l
= pc
Compton Wavelength:
h
lC =
= 0.00243nm
me c
is the characteristic change in
wavelength in the scattered photon.
What is matter?
Let’s Shoot stuff at it to find out…
Rutherford-Bohr Model of the Atom
• Ernest Rutherford (1871-1937)
• Rutherford Scattering
Observation consistent with scattering
from a very small (10,000 times smaller
radius than the atom) dense object… The
nucleus
Models of the Atom
• Greek Model of the Atom. Democritus, Leucippus et al
• “Plum Pudding” model of the atom. J.J. Thomson et al.
• Classical solar system model.
• Bohr Model of the atom.
• De Broglie Model of the atom.
• Schroedinger Model of the Atom (english)
•
http://phet.colorado.edu/sims/hydrogen-atom/hydrogen-atom.jnlp
Wavelengths of Hydrogen
Bohr’s Semi-classical Model of the Atom
• Bohr assumed for the
electron proton system
– to be subject to Coulomb’s
Law for electric charges
– Quantization of angular
momentum for the electron
orbit. L=nh/2
• Quantization condition
prevents electron from
continuosly radiating away
energy
• Discrete energy levels for
electron orbit
Bohr’s Semi-classical Model of the Atom
Bohr’s Semi-classical Model of the Atom
Bohr’s Semi-classical Model of the Atom
Bohr’s Semi-classical Model of the Atom
Bohr’s Semi-classical Model of the Atom
Bohr Hydrogen atom and spectral lines
Quantum Mechanics and Wave-Particle
Duality
• QM Tunneling
• DeBroglie Matter
Wave
• Heisenberg
Uncertainty Principle
• Schroedinger
Equation and QM
atom
• Spin and Pauli
Exclusion Principle
DeBroglie Matter Wave
Quantum Mechanics and Wave-Particle
Duality
• Fourier
• Wave Packets
Uncertainty Principle
Uncertainty Principle
Uncertainty Principle
Schrodinger Wave Equation and
Hydrogen Atom
Additional Quantum Numbers and
splitting of spectral lines
• Normal Zeeman Effect
Can measure magnetic fields by
examining spectra!!!!
Spin and the Pauli Exclusion Principle
No two electrons can share the same set of four
quantum numbers
Chemistry….The periodic table….
Electron Degeneracy Pressure and White
Dwarfs
• Exclusion Principle for
Fermions (spin-1/2 particles)
and uncertainty principle
provide electron degeneracy
pressure that is the
mechanism that prevents the
further collapse of white
dwarves….
The Complex Spectra of Atoms
• Number of possible energy
levels increases with
number of electrons.
n,l,m,…
• Interactions with external
magnetic fields, etc…
Complicated Spectra!!!!
Links
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http://sci2.esa.int/interactive/media/flashes/2_2_1.htm
http://wps.prenhall.com/wps/media/objects/610/625137/Chaisson/CH.00.002/HTML/C
H.00.002.s5.htm
http://phet.colorado.edu/simulations/sims.php?sim=Photoelectric_Effect
http://www.student.nada.kth.se/~f93-jhu/phys_sim/compton/Compton.htm
http://phet.colorado.edu/sims/rutherford-scattering/rutherford-scattering.jnlp
http://phet.colorado.edu/sims/hydrogen-atom/hydrogen-atom.jnlp
http://www.nobelprize.org/nobel_prizes/physics/laureates/1929/broglie-lecture.pdf
http://adsabs.harvard.edu/cgi-bin/nphabs_connect?fforward=http://dx.doi.org/10.1002/andp.19263851302
http://web.archive.org/web/20081217040121/http://home.tiscali.nl/physis/HistoricPape
r/Schroedinger/Schroedinger1926c.pdf
http://phet.colorado.edu/sims/quantum-tunneling/quantum-tunneling.jnlp
http://phet.colorado.edu/sims/fourier/fourier.jnlp