Basics of Interplanetary Flight


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Spectroscopy

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Data from optical spectroscopic instruments can lead to knowledge of many of a light source's properties. Depending on the nature and condition of the source this may include temperature, pressure, density, luminosity, chemical composition, state(s) of ionization, rotation rate, relative velocity, mass (under some specific circumstances), and the composition and density of material intervening between the source and the instrument.

Consider how many wavelengths or colors comprise white light. Given its strength here on Earth, sunlight can be broken down into its constituent wavelengths with enormously fine resolution; thousands of individual wavelengths and meaningful features can be measured in the solar spectrum.

Optical spectroscopic instruments have led to many important findings. A few examples include:

  • The discovery and naming of a mysterious, light-absorbing substance in the Sun ("helium") before it was identified on Earth;

  • Compositions of solar system objects' atmospheres and surfaces;

  • Clouds containing complex organic compounds in interstellar space;

  • The expansion of the universe;

  • Confirmation of liquid on the surface of Saturn's moon Titan.


Compare:

  • Spectroscope,

  • Spectrometer,

  • Spectrograph,

  • Spectroscopy.


Kirchhoff's three laws of spectroscopy

  • A hot solid object produces light with a continuous spectrum.

  • A hot tenuous gas produces light with spectral lines at discrete wavelengths (colors) which depend on the energy levels of the atoms in the gas.

  • A hot object surrounded by a cool tenuous gas (i.e. cooler than the hot object) produces light with an almost continuous spectrum which has gaps at discrete wavelengths depending on the energy levels of the atoms in the gas.
Kirchhoff did not know about energy levels in atoms. The existence of discrete spectral lines was later explained by the Bohr model of the atom, which helped lead to quantum mechanics.



Activity with Absorption Spectra

  • Examine a hardcopy of a high-resolution solar spectrum.

  • Identify absorption lines (dark gaps) and consider their origin.


Activity with Emission Spectra
Refer to the image at right (click to enlarge):

  • Experiment with a makeshift DVD spectroscope, a white light source, and a neon bulb.

  • Observe the light emitted by each of various hot-gas lamps, using spectral gratings to observe emission lines.

  • If you're operating the lamps, please observe 30 seconds maximum ON time with at least 30 seconds OFF time.

  • Match the emission-line "fingerprints" that you observed with samples at right, thus identifying the gas in the lamp.



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PAGE UPDATED UTC 2009 JULY 1   -dd