Spectral Lines

We have seen how a blackbody spectrum is related to the temperature of a body.  This smoothly varying spectrum is called a kontinuum spectrum because it is made up of all possible frequencies.  Spectral lines, on the other hand, are narrow features superimposed on the continuum spectrum

• Spectral lines are seen when light is passed through a slit, then expanded into a spectrum.  A device for doing this is called a spectroscope.
• Absorption lines are dark lines due to "missing" frequencies
• Emission lines are bright lines due to extra emission at some frequencies
• Glowing gas shows emission lines, but if you put this same gas in front of a bright source of light, the same lines appear in absorption.
• For gas of different composition, the spectral lines appear in different places, unique for each element.

• Fig. Absorption lines

• Fig. Emission lines

SPECTRAL LINES

Spectral lines and spectrum

The spectral line intensities are related to the temperature of the body doing the emitting.  In the case of the 5000 K gas in front of the 6000 K background, the background has a normal Planck Function blackbody spectrum except where the cooler gas is absorbing it.  The depth of the lines reflect the 5000 K blackbody spectrum of the gas.  In the case of the 5000 K gas with a cool background, the height or intensity of the spectral lines reflects the 5000 K blackbody curve of the gas, but only in the spectal lines.  At other wavelengths, the gas has no emission, and so is dark.

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Measuring Spectra

Spectra are measured in wavelength, either in angstrom (1 A = 10-10 m), or nanometer (1 nm = 10-9 m) units.  The use of nm is more common in scientific literature, but angstroms are used often in ordinary speech.  Visible light ranges from about 4000 A (blue) to 8000 A (red), or 400 nm - 800 nm.