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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 l.jpg
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)


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Spectroscopy

29 September 2006


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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?


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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)





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Emission Spectra (“nonthermal”)from hot, transparent gases


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Hydrogen emission spectrum(“Balmer lines”)






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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






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What if a wave source is moving?

“Doppler effect”


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Doppler-shifted absorption spectra

% shift in wavelength = speed of source,

as % of speed of light


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Doppler-shifted absorption spectra

% shift in wavelength = speed of source,

as % of speed of light


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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)


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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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