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Ch 04--Origin and Nature of Light
21 Sep 2000
ASTR103, GMU, Dr. Correll
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Ch 04--Origin and Nature of Light
• Blackbody Radiation
• Discovering Spectra
• Atoms and Spectra
21 Sep 2000
ASTR103, GMU, Dr. Correll
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Blackbody Radiation
• “Glowing red hot”--blackbody radiation is the
name given to electromagnetic radiation emitted
by an heated object.
– Solids and dense gases give off blackbody radiation
21 Sep 2000
ASTR103, GMU, Dr. Correll
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E&M Radiation
• All matter is constantly in motion at the
atomic level
• The higher the temperature, the more
motion
• The more motion, the more light that is
radiated
Let’s digress to consider atomic theory...
21 Sep 2000
ASTR103, GMU, Dr. Correll
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Atomic Theory
Atom - smallest unit displaying particular chemical and
physical properties
Ernest Rutherford (1871-1937) - atom mostly empty
space
Nucleus contains 99.98% of mass
Nucleus - central component of atom
Size - about 10-4 of radius of electron orbits
Mass - about 2000 times that of electron
Density - about 1012 to 1014 g/cm3
Electron clouds - clusters of electron orbits encircling
nucleus
21 Sep 2000
ASTR103, GMU, Dr. Correll
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Atomic Theory (cont.)
• Electron identified 1897, J. J. Thomson (1856-1940)
– Unit of negative electrical charge
– Mass - about 1/2000 that of proton
• Light mass makes them fast!
• Proton identified 1919 by Rutherford as principal
constituent of nucleus
– Unit of positive electrical charge
– Mass - 2000 times that of electron
• Neutron identified 1932 by James Chadwick (18911974) as second primary particle in nucleus
– No net electrical charge
– Mass - approximately that of proton
21 Sep 2000
ASTR103, GMU, Dr. Correll
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Atomic Theory (cont.)
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ASTR103, GMU, Dr. Correll
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Electromagnetic Force
• Lorentz Force
– q is charge for particle
one and two
– r is seperation
– k is constant of
proportionality
– notice negative sign!
• Does this equation look
familiar?
21 Sep 2000
ASTR103, GMU, Dr. Correll
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How big is an atom
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ASTR103, GMU, Dr. Correll
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Elements--different kinds of atoms
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ASTR103, GMU, Dr. Correll
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States of Matter
• Solids - constituents, molecules or atoms,
maintain reasonably permanent relation to each
other
– Typical separation is few constituent diameters
– Solids rare in Universe
21 Sep 2000
ASTR103, GMU, Dr. Correll
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States of Matter (cont.)
• Liquids - constituents, molecules or atoms,
maintain only temporary relation to each other
– Typical separation is several constituent diameters
– Liquids are non-existent for all practical purposes
21 Sep 2000
ASTR103, GMU, Dr. Correll
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States of Matter (cont.)
• Gases - constituents, molecules or atoms,
maintain no relation relative to each other
– Typical separation is many constituent diameters
– Gases common in Universe
21 Sep 2000
ASTR103, GMU, Dr. Correll
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What about Ions?
-
Ion: Helium  He+  He II
Nucleus: Helium 4  4He2
-
+
o
electron
proton
neutron
+
o
+
o
Nucleus
One electron system
21 Sep 2000
ASTR103, GMU, Dr. Correll
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States of Matter (cont.)
• Plasmas - state similar to gases, but atoms are
ionized
– One or more electrons stripped off atom
– Most visible matter in Universe in form of a plasma
– Highly ionized plasmas predominate
21 Sep 2000
ASTR103, GMU, Dr. Correll
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Blackbody Radiation
• Planck’s Law
– 1900, Max Planck derived mathematical law describing
distribution of brightness in blackbody spectrum
• Stefan-Boltzmann Law
– Energy emission is greater at every wavelength as
temperature increases; total amount of radiant energy
emitted increases with increasing temperature
• Wien’s Displacement Law
– Maximum emission found toward shorter wavelengths
(blue end of spectrum) as temperature increases
21 Sep 2000
ASTR103, GMU, Dr. Correll
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Radiation Laws
• Planck’s Law………………
• Stefan-Boltzmann Law…..
• Wien’s Displacement Law.
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ASTR103, GMU, Dr. Correll
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Blackbody Radiation
• Planck’s Law
• StephanBoltzmann Law
• Wien’s
Displacement
Law
21 Sep 2000
ASTR103, GMU, Dr. Correll
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Blackbody Radiation (cont.)
• Radiation emitted
by stars tends to be
much like that
emitted by
blackbody
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ASTR103, GMU, Dr. Correll
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Blackbody Radiation (cont.)
21 Sep 2000
ASTR103, GMU, Dr. Correll
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Ch 04--Origin and Nature of Light
• Blackbody Radiation
• Discovering Spectra
• Atoms and Spectra
21 Sep 2000
ASTR103, GMU, Dr. Correll
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Discovering Spectra
• Fraunhofer lines in the solar spectrum (1814)
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ASTR103, GMU, Dr. Correll
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Kirchoff-Bunsen Experiment
• Different
chemicals have
different spectra!
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ASTR103, GMU, Dr. Correll
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Spectrometry
• Spectrometry--a very
important tool in
astronomy!
– Spectrum recorded at the
focal plane of a telescope
– spectra give information
about the composition,
temperature and
pressure of the
astronomical object
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ASTR103, GMU, Dr. Correll
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Spectrum
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ASTR103, GMU, Dr. Correll
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Spectra
21 Sep 2000
ASTR103, GMU, Dr. Correll
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Ch 04--Origin and Nature of Light
• Blackbody Radiation
• Discovering Spectra
• Atoms and Spectra
21 Sep 2000
ASTR103, GMU, Dr. Correll
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Atoms and Spectra
• Until now, we’ve talked about atoms as
little billiard balls--nuclei dragging
electrons around to produce E&M
radiation
• But the structure we see in the spectra
of light indicates that the structure of
these atoms has some interesting
features
– This leads to the quantum theory of the
atom!
21 Sep 2000
ASTR103, GMU, Dr. Correll
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Bohr Model of the Atom
• Bohr Model--Bohr
hypothesized that
electrons orbit at
discrete levels,
jumping up or down
in energy levels
(1911)
– Planck and Einstein
had earlier proposed
quantum ideas about
light
21 Sep 2000
ASTR103, GMU, Dr. Correll
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Bohr Model of the Atom
• Electrons change
energy levels in
an atom by
absorbing or
emitting a photon!
• Electrons tend to
settle to the
lowest energy
level, the ground
state
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ASTR103, GMU, Dr. Correll
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Hydrogen, for example
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ASTR103, GMU, Dr. Correll
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Doppler Shift
• What happens to light when source and observer
move relative to each other? Doppler shifting of
frequency!
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ASTR103, GMU, Dr. Correll
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Doppler Shift
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ASTR103, GMU, Dr. Correll
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Doppler Shift
• Motion of source
away form observer
causes a red shift
• Motion of source
towards observer
causes a blue shift
• Motion lateral to
observer gives no
shift!
21 Sep 2000
ASTR103, GMU, Dr. Correll
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