Lecture 25 Atoms and Spectral Lines

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Lecture 25Atoms and Spectral Lines
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Atoms and Spectral Lines
The Atom
Nucleus (protons and neutrons)
n1
n2
Low energy orbit
Protons charge Electrons - charge Neutrons
neutral
n3
High energy orbit
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Atoms

• The type of element depends on the number of
  protons in the nucleus.
• Hydrogen 1 proton
• Helium 2 protons
• Carbon 6 protons
• of electrons usually equals number of protons
• Atom is electrically neutral
• Ion atom where number of electrons does not
  equal number of protons

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Electrons

• Electrons can move to different orbits, but they
  must gain or lose energy to do so.
• Energy difference between levels is unique for
  each element and corresponds to wavelength of
  light.

High energy level
Low energy level
n1
n2
n3
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Absorption Lines
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Absorption Spectra

• Continuum radiation observed through cool gas
• Electron absorbs a photon of light to move to a
  higher energy level

Photon energy must be equal to energy difference
in atom
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Emission Lines
Intensity
Wavelength
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Emission Spectra

• Observe a hot, thin gas
• Electron gives off a photon of light to move to a
  lower energy level

Photon energy must be equal to energy difference
in atom
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Ionization

• Electron absorbs enough energy from photon to
  leave the atom

photon
Intensity
n1
n2
Wavelength
n3
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Recombination

• Electron is recombines with the atom and gives
  off energy

photon
Intensity
n1
Wavelength
n2
n3
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Review of Spectra
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Doppler Shift

• The wavelength of light is affected by the motion
  of the object.

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• Object moving toward observer wavelength gets
  shorter (blue-shifted)

• Object moving away from observer wavelength gets
  longer (red-shifted)

• NOTE We only see motion along the line of sight
  -- side to side motion cannot be measured.

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Spectrum of Saturns Rings
Spectrum of ice when at rest
A
B
C