Kinds of Spectra

    There are several kinds of spectra, such as: continuous; bright line; and dark line.

    A continuous spectrum is, as the name implies, a parade of all the colors from the deepest red to the ultraviolet - of which the rainbow in the sky is a good example.  In the laboratory a continuous spectrum can be produced by heating a solid, a liquid, or an opaque gas to a fairly high temperature - several thousand degrees Fahrenheit.  Light from the electric lamp filament, for example, produces such a spectrum.

Continuous Spectrum

    When light emitted by a gas through which an electrical discharge is passing produces a spectrum consisting of a few isolated parallel lines, it is known as a "bright line spectrum" or "emission spectrum".   

Emission spectrum of hydrogen (bright line spectrum)

   The spectrum produced by hydrogen consists of only several bright lines, on a black background.

    Note that the shape of the "bright line" is due to the narrow slit.  Had the slit been in the shape of a crescent, the lines would be crescent-shaped.

    The characteristic color of neon signs is due to bright red and orange lines in its spectrum.

The typical color of neon signs is due to the great
intensity of the red and orange lines; these
overshadow the other colors present in the spectrum.
This is a negative of the spectrum seen.  The numbers
are in Angstrom units

    Bright lines of any element can also be produced by placing a volatile salt of that element in a flame.

    The spectrum due to sodium vapors shows only one visible yellow line against a dark background.  On closer examination, the yellow line turns out to be a doublet, that is, two very close lines.

Sodium Spectrum
The yellow light typical of sodium lamps is due
primarily to the two very bright lines in the yellow
region of the spectrum. 

    It should be carefully noted that each element always gives the same pattern of lines.  Each element, so to speak, has its own fingerprints, possessed by no other element.  This fact is utilized in chemical analysis and in many other applied fields.

    Dark line spectrum (or absoption spectrum) is due to absorption of light of particular wave length by relatively cool gases.  The wave lengths absorbed are identical to the wave lengths that the gas would emit when properly excited.

Photons of the proper wavelengths can be absorbed by the gas
atoms and re-emitted in random directions.  Because most
of these particular photons do not reach the telescope,
the spectrum is dark at the wavelengths of the missing photons.

Absorption spectrum of hydrogen (dark-line spectrum)

    Stellar spectra are dark line spectra.  Some of the dark lines are due to hydrogen, others to helium, still others to other elements.  From a careful study of these lines, the chemicals present on a star, as well as other information, are obtained.

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