Novice Astronomy / Physics Question, wrt spectroscopes.

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From: (E Schlafly)
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Subject: Novice Astronomy / Physics Question, wrt spectroscopes.
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Date: 27 Jul 2003 01:36:15 -0700
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Organization: http://groups.google.com/

You asked the same questions Kirchoff did in the mid 1800s! From
these questions he wroked up the three kinds of spectra: bright line,
dark line, and cintinuous. You explanation of the bright line
spectrunm is more or less correct. Start with that.
The continuous spectrum is a blackbody spectrum, first hinted at
by John Draper (homeboy from Greenwxih Village, Manhattan). The
thermal agitation of the gas atoms radiate at all wavelengths in a mix
given by Plank's rquation. This is the humpback curve or continuum you
see in spectrodensitometer tracings.
The Sun gives off, by its temperature, a blackbody continuum. That
radiation passes thru other gases above the Sun's surface. Certain of
the wavelengths in that blackbody radiation are the exact ones to
excite the atoms in this superincumbent gas. This gas accepts the
radiation at these peculiar wavelengths, And there by removes those
wavelengths from the continuum. Result is that above the Sun's outer
gases, the raidation consists of the continuum laced by dark lines at
the wavelengths knocked out of it.



ES I found myself perplexed recently by spectroscopes, and would
ES appreciate it if someone wiser than I out there would clear up my
ES confusion. Probably this stuff is well understand and not deserving
ES of being mentioned here; if you could then just provide me with a few
ES good words for search terms to find relevant data it'd be great.
ES
ES As I understand it, light is emitted by things as electrons in excited
ES energy levels fall down into lower, standard energy levels. This
ES nicely explains why looking at many things through a spectroscope
ES reveals a few bright wavelengths of light in a sea of darkness.
ES
ES It makes it non-obvious, however, why the sun produces a large, mostly
ES continuous band of colors. One site suggested that this occurs in
ES heated gases of high pressure, while the former phenomenon occurs in
ES heated gases of low pressure, but supplied no reasoning, and this
ES behavior is not self-evident to me.
ES
ES My last point of confusion (to be mentioned here!) surrounds the gaps
ES in the "mostly continuous band of colors" mentioned above. As I
ES understand it, these gaps represent absorbed / scattered wavelengths
ES of light that correspond to the wavelengths of light emitted by heated
ES gases as discussed two paragraphs above. This makes sense; I just
ES don't understand how a _single_ wavelength of light's being missing
ES could make an overall difference in the spectrograph's shown spectrum.
ES It would seem to me that the infinitely(?) large number of
ES wavelengths produced would make distinguishing a single one
ES impossible, much as seeing a single missing, infinitely small point on
ES a line would not be possible. Can an electron of an element's atom be
ES excited by a whole, albeit very small, range of wavelengths / energies
ES of light, or just a single one that composes an infinitely small
ES portion of the overall spectrum?
ES
ES In other words, if the dark line represents only one wavelength, how
ES is this gap appreciable in a spectrum consisting of an infinite(?)
ES number of wavelengths?

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