CoachA gas at low pressure, excited by an electric discharge (as in a neon tube or a sodium street lamp), glows too — but through a spectroscope its light is nothing like a rainbow. Only a few isolated wavelengths are present: thin bright lines on a black background, called spectral lines. Such a spectrum is an emission line spectrum.
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Example 2.11 (Hydrogen and sodium)
Excited hydrogen emits exactly four lines in the visible: a red line at , a blue-green line at , and two violet lines at and . Excited sodium emits essentially one strong yellow-orange line at — which is why older street lamps bathe whole neighborhoods in that single color: their light contains almost nothing else.