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Announcements. Homework 5 due Monday Solutions to test are posted near south entrance to Science Lab building (until next test) Second test will be next week (W, Th) Looks like a great weekend for projects! (Due two weeks from today). Spectroscopy. 29 September 2006. Today:.

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announcements
Announcements
  • Homework 5 due Monday
  • Solutions to test are posted near south entrance to Science Lab building (until next test)
  • Second test will be next week (W, Th)
  • Looks like a great weekend for projects! (Due two weeks from today)
spectroscopy

Spectroscopy

29 September 2006

today
Today:
  • Splitting light into separate colors (wavelengths)
  • Spectra of thermal light sources
  • Spectra of nonthermal light sources
  • Absorption spectra
  • How does motion of a light source affect its spectrum?
thermal light sources hot opaque objects
Thermal Light Sources(Hot, opaque objects)
  • Emit a continuous spectrum (all colors present)
  • Hotter implies brighter
  • Hotter implies bluer in color
  • Brightness & color don’t depend much on what the object is made of
  • Examples: incandescent light filaments (3000 K); electric heating coils (1500 K); coals in a campfire (1500 K); your body (310 K); the sun minus its outer layers (6000 K)
nonthermal light sources especially hot transparent gases
Nonthermal Light Sources(Especially hot, transparent gases)
  • Emit only a few precise wavelengths (colors)
  • Temperature matters less than chemical composition
  • Each element has its own spectral “signature”
  • Examples: mercury and sodium vapor lights; lasers (just one wavelength); interstellar gas clouds
doppler shifted absorption spectra
Doppler-shifted absorption spectra

% shift in wavelength = speed of source,

as % of speed of light

doppler shifted absorption spectra21
Doppler-shifted absorption spectra

% shift in wavelength = speed of source,

as % of speed of light

doppler shifted absorption spectra22
Doppler-shifted absorption spectra

Shorter wavelengths implies it’s moving toward us

Wavelengths are shifted by about 10 units (Angstroms) out of 4000, or 1 part in 400. Therefore this object is moving toward us at 1/400 the speed of light (750 km/s)

the physics of light
The Physics of Light
  • Speed = 300,000 km/s
  • Brightness of a source is measured in watts (a unit of power, energy/time)
  • Diffraction indicates wavelike behavior
  • Made of tiny units called photons
  • Wavelength determines color and photon energy
  • Hot, opaque object emits continuous spectrum, brighter and bluer if hotter
  • Hot, transparent gas emits bright-line spectrum
  • Thermal source viewed through cooler gas has dark-line “absorption” spectrum
  • Doppler-shifted spectral lines indicate motion of source toward or away from us
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