Physics 320: Astronomy and Astrophysics

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Physics 320 Astronomy and Astrophysics Lecture
V

• Carsten Denker
• Physics Department
• Center for SolarTerrestrial Research

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The Interaction of Light and Matter

• Spectral Lines
• Photons
• The Bohr Model of the Atom
• Quantum Mechanics and WaveParticle Duality

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Electromagnetic Spectrum
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Spectral Lines

• Auguste Comte 1835 in Positive Philosophy We see
  how we may determine their forms their distances,
  their bulk, their motions, but we can never know
  anything of their chemical or minerological
  structure.
• William Wollaston, Joseph Fraunhofer, Robert
  Bunsen, Gustav Kirchhoff, ? spectroscopy

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Kirchhoffs Laws

• A hot (lt 0 K), dense gas or solid object produces
  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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Spectroscopy

• Prisms
• Diffraction gratings
• Transmission grating
• Reflection grating

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Photoelectric Effect
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Compton Effect
In a collision between a photon and an electron
initially at rest, both the (relativistic)
momentum and energy are conserved.
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The Bohr Model of the Atom

• Waveparticle duality of light
• Rutherford 1911 ? ? Au It was quite the most
  incredible event that ever happened to me in my
  life. It was almost as incredible as if you fired
  a 15inch shell at a piece of tissue paper and it
  came back an hit you. ? discovery of a minute,
  massive, positively charged atomic nucleus
• Proton mp 1836 ? me

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Group AssignmentProblem 5.7

• Verify that the units of Plancks constant are
  the units of angular momentum!

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Hydrogen Atom
Planetary model of the hydrogen atom?
m 1 UV 122, 103, 97, nm Lyman
m 2 Visible 656, 486, 434, nm Balmer
m 3 IR 1875, 1282, 1094, nm Paschen
m 4 IR 4051, 2625, 2165, nm Brackett
m 5 IR 7458, 4652, nm Pfundt
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Bohrs Postulates

• Only orbits are stable, where the angular
  momentum of the electron is quantized L
  nh/2?nh, and will not radiate in spite of the
  electrons acceleration.
• Every allowed orbit corresponds to a distinct
  energy level and the transition from a distant
  orbit to an orbit closer to the nucleus Ephoton
  Ehigh Elow results in the emission of an energy
  quantum, i. e., a photon.

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Bohr Atom
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(No Transcript)
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Bohr Atom (cont.)
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Kirchhoffs Laws Revisited

• A hot, dense gas or hot solid object produces a
  continuous spectrum with no dark spectral lines.
  This is the continuous spectrum of black body
  radiation, described by the Planck functions
  B?(T) and B?(T), emitted at any
  
  temperature above
  
  absolute zero. The
  
  wavelength ?max
  at
  
  which the Planck
  
  function B?(T) obtains
  
  its maximum
  is given
  
  by Wiens
  
  
  displacement law.

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Kirchhoffs Laws Revisited (cont.)

• A hot, diffuse gas produces bright emission
  lines. Emission lines are produced when an
  electron makes a downward transition from a
  higher to a lower orbit. The energy lost by the
  electron is carried away by the photon.
• A cool, diffuse gas in front of a source of
  continuous spectrum produces dark absorption
  lines in the continuous spectrum. Absorption
  lines are produced when an
  electron
  makes a transition from a lower
  to a
  higher orbit. If the incident photon in
  
  the continuous spectrum has exactly the
  right
  amount of energy, equal to the
  difference in
  energy between a higher
  orbit and the
  electrons initial orbit, the
  photon is
  absorbed by the atom and the
  
  electron makes an upward transition to
  the higher
  orbit.

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Quantum Mechanics and
WaveParticle Duality
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Problem 4.5
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Problem 4.13
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Problem 4.18
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Homework Class Project

• Prepare a 200 250 word abstract for one of the
  five topics mentioned in the storyline
• Important scientific facts
• Form of presentation
• Learning goals
• Homework is due Wednesday October 8th, 2003 at
  the beginning of the lecture!
• Exhibition name competition!

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Homework

• Homework is due Wednesday October 8th, 2003 at
  the beginning of the lecture!
• Homework assignment Problems 5.4, 5.5, and 5.15
• Late homework receives only half the credit!
• The homework is group homework!
• Homework should be handed in as a text document!