Doppler Effect

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Doppler Effect

• The Doppler Effect is the motion induced change
  in the observed frequency of a wave.
• The effect can only be observed/seen due to the
  relative motion between the observer and the wave
  source.
• A net motion towards the source causes a
  blueshift a shift to higher frequencies - in
  the received beam. A net motion away from the
  source causes a redshift.
• The degree/ extent of the shift is directly
  proportional to the observers velocity relative
  to the source.

http//webphysics.davidson.edu/Applets/Applets.htm
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Doppler again ?
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Spectroscopy

• Recall - atom/molecule emits radiation in the
  form of photons.
• This radiation can be picked by a spectroscope, a
  device that disperses this radiation.
• Spectroscopes have diffraction gratings that act
  like prisms.

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Spectroscopy

• 3 types of spectra 1. continuous, 2.
  emission and 3. absorption.

• Kirchhoffs Laws
• P. 89

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Spectroscopy and Sources.
1. A luminous solid or liquid or a sufficiently
dense gas.
2. Low density hot gas.
3. A cool thin gas absorbs certain wavelengths
from a continuous spectrum.
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Actual Spectra
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Spectra from
The Sun
Numbers are in nanometers
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Spectral Information

• Doppler effect and line broadening when a
  familiar pattern of lines appears, but the lines
  are displaced from their usual locations.
• Thermal Broadening -gt

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Spectral Information

• Rotation Broadening.

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Spectral Information

• Wiens law wavelength of peak emission
  1/Temperature.
• What this says is that the hotter the object the
  bluer its radiation and vice versa.
• Temperature is measured in Kelvins (K) where
  kelvins degrees Celsius 273 (page 73).
• Temperature is a direct measure of the amount of
  microscopic motion within an object the hotter
  the object, that is the higher its temperature,
  the faster its constituent particles move and the
  more energy they radiate.

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Summary of spectral information.
Peak frequency or wavelength (continuous spectra only) Temperature (Wiens law)
Lines Composition, temperature
Line width Temperature, turbulence, rotation speed, density, magnetic field
Doppler shift Line-of-sight velocity