Chapter 5: Light: The Cosmic Messenger

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Chapter 5Light The Cosmic Messenger
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What is Light?

• Light is radiative energy
• Energy is measured in Joules
• Power is measured in Watts 1 watt 1 joule/s
• The power a person uses in a day is about 10
  Mjoules, equivalent to leaving a 100W bulb on all
  day

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How Light Behaves

• Emission
• Absorption
• Transmission
• Reflection

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If you pass white light through a prism, it
separates into its component colors.
R.O.Y. G. B.I.V
spectrum
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Duality of Light

• Light can behave as a particle and a wave
• In the 17th Century, Isaac Newton argued that
  light was composed of little particles while
  Christian Huygens suggested that light travels in
  the form of waves.
• In the 19th Century, Thomas Young demonstrated
  that light bends slightly around corners and acts
  like interfering waves.

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Thomas Youngs interference experiment
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Scottish physicist James Clerk Maxwell showed
mathematically in the 1860s that light must be a
combination of electric and magnetic fields.
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Light and Energy

• Photon energy Planks constant x speed of light
  / wavelength E h x f hc/?
• As Energy goes up, frequency goes up, wavelength
  gets smaller

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Peak color (wavelength) shifts to shorter
wavelengths as an objects is heated
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Peak color (wavelength) emitted depends on an
objects temperature
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Peak color (wavelength) shifts to shorter
wavelengths as an objects is heated
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The intensities of different emitted colors
reveal a stars temperature

• Wiens Law
• Wavelength is inversely proportional to
  temperature
• lmax (2.9 x 10-3) / TKelvin
• As Energy goes up, Temperature increases and
  wavelength gets smaller, and frequency gets
  greater

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What color is our 5800K Sun?

• The Sun emits all colors (wavelengths of
  electromagnetic radiation) however, the colors
  it emits most intensely are in the blue-green
  part of the spectrum.

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Each chemical element produces its own unique set
of spectral lines when it burns
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The Suns Spectrum
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The brightness of spectral lines depend on
conditions in the spectrums source

• Law 1 A hot object or a hot, dense gas produces
  a continuous spectrum -- a complete rainbow of
  colors with without any specific spectral lines.
  (This is a black body spectrum.)
• Law 2 A hot, rarefied gas produces an emission
  line spectrum - a series of bright spectral lines
  against a dark background.
• Law 3 A cool gas in front of a continuous source
  of light produces an absorption line spectrum - a
  series of dark spectral lines among the colors of
  the rainbow.

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Absorption Spectrum of Hydrogen Gas
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Kirchhoffs Laws
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Spectral lines occur when an electron jumps from
one energy level to another
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We can determine a stars movement by observing
the shift in spectrum of the light from a star

• Barnards Star

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Spectral lines shift due to the relative motion
between the source and the observer
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Doppler Shifts

• Red Shift The distance between the observer and
  the source is increasing
• Blue Shift The distance between the observer and
  the source is decreasing

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The Doppler shift allows astronomers to measure
radial velocity
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What can we learn by analyzing starlight?

• A stars temperature
• by peak wavelength
• A stars chemical composition
• by spectral analysis
• A stars radial velocity
• from Doppler shifts