Post on 26-Feb-2018
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Lecture Outlines
Astronomy Today
7th Edi t ion
Chaisson/McMillan
2011 Pearson Education, Inc.
Chapter 4
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2011 Pearson Education, Inc.
Chapter 4
Spectroscopy
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2011 Pearson Education, Inc.
4.1Spectral Lines
4.2Atoms and Radiation
The Hydrogen Atom
The Photoelectric Effect
4.3The Formation of Spectral Lines
4.4Molecules
4.5Spectral-Line Analysis
Units of Chapter 4
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Spectroscope: Splits light into componentcolors
4.1 Spectral Lines
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Emission lines:
Single frequencies
emitted by
particular atoms
4.1 Spectral Lines
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Emission spectrumcan be used to identify
elements
4.1 Spectral Lines
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Absorption spectrum:If a continuous spectrum
passes through a cool gas, atoms of the gas willabsorb the same frequencies they emit
4.1 Spectral Lines
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An absorption spectrum can also be used toidentify elements. These are the emission and
absorption spectra of sodium:
4.1 Spectral Lines
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Kirchhoffs laws:
Luminous solid, liquid, or dense gas
produces continuous spectrum
Low-density hot gas produces emissionspectrum
Continuous spectrum incident on cool, thin
gas produces absorption spectrum
4.1 Spectral Lines
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Kirchhoffs laws illustrated
4.1 Spectral Lines
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Existence of spectral lines required new
model of atom, so that only certain amountsof energy could be emitted or absorbed
Bohr modelhad certain allowed orbits for
electron
4.2 Atoms and Radiation
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Emission energiescorrespond to energy
differences between allowed levelsModern model has electron cloudrather than
orbit
4.2 Atoms and Radiation
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Energy levelsof the hydrogen atom, showing
two series of emission lines:
More Precisely 4-1: The Hydrogen Atom
The energies of
the electrons in
each orbit are
given by:
The emission
lines correspond
to the energy
differences
2
113.6 1 -- eV.nE
n
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The photoelectric effect:
When light shines on metal, electrons can be
emitted
Frequencymust be higher than minimum,characteristic of material
Increased frequencymore energetic
electrons
Increased intensitymore electrons, same
energy
Discovery 4-1: The Photoelectric Effect
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Photoelectric effectcan only be understood if
light behaves like particles
Discovery 4-1: The Photoelectric Effect
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Light particles each have energy E:
Here, his Plancks constant:
4.2 Atoms and Radiation
E hf
-34
6.63 10 joule seconds (J s)h
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Absorption can boost an electron to thesecond (or higher) excited state
Two ways to decay:
1. Directly to ground state
2. Cascadeone orbital at a time
4.3 The Formation of Spectral Lines
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(a) Direct decay (b) Cascade
4.3 The Formation of Spectral Lines
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Absorption spectrum: Created when atoms
absorb photons of right energy for excitation
Multielectron atoms: Much more complicated
spectra, many more possible states
Ionizationchanges energy levels
4.3 The Formation of Spectral Lines
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Emission linescan be used to identify atoms
4.3 The Formation of Spectral Lines
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Molecules can vibrate and
rotate, besides havingenergy levels
Electron transitions
produce visible andultraviolet lines
Vibrational transitions
produce infrared linesRotational transitions
produce radio-wave lines
4.4 Molecules
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Molecular spectraare much more complex
than atomic spectra, even for hydrogen
(a) Molecular hydrogen (b) Atomic hydrogen
4.4 Molecules
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Information that can be gleaned from
spectral lines:
Chemical composition
Temperature
Radial velocity
4.5 Spectral-Line Analysis
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Line broadening
can be due to a
variety of causes
4.5 Spectral-Line Analysis
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4.5 Spectral-Line Analysis
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4.5 Spectral-Line Analysis
The Doppler shift may
cause thermalbroadening of spectral
lines
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4.5 Spectral-Line AnalysisRotation will also cause broadening of
spectral lines through the Doppler effect
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Spectroscope splits light beam intocomponent frequencies
Continuous spectrum is emitted by solid,
liquid, and dense gas
Hot gas has characteristic emission spectrum
Continuous spectrum incident on cool, thin
gas gives characteristic absorption spectrum
Summary of Chapter 4
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Spectra can be explained using atomicmodels, with electrons occupying specific
orbitals
Emission and absorption lines result from
transitions between orbitals
Molecules can also emit and absorb
radiation when making transitions between
vibrational or rotational states
Summary of Chapter 4 (cont.)