Emission Spectra

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When heated, materials with electrons that are not confined within individual atoms or molecules (so-called "free electrons") produce radiation at all wavelengths (since the electrons can make any change that is consistent with their total energy). Such free electrons are found in solids and plasmas (ionized gases). When a solid object (such as the stove burner in the photo above) is heated sufficiently, it will produce radiation in the visible range of wavelengths and will appear as a "rainbow" or, technically, a "continuous spectrum". The most intense (brightest) part of this spectrum is determined by the exact temperature of the object. As a stove burner is considered to be relatively cool, the red end of the spectrum is much more intense than the higher-energy blue end.
When electrons are bound to individual atoms and molecules (as in gases), they can only change their energies by amounts dictated by the structure of the particular atom or molecule. In such cases, the radiation will be restricted to a few wavelengths and produces what is called a "line spectrum". The atoms in the gas cloud in the photo above are colliding with the surrounding material, "bumping" the electrons up to higher energy states in the atoms. The electrons then return to the ground state by emitting radiation. Since the set of possible energy states for the electrons is limited, they can produce only a limited set of wavelengths (the "emission line spectrum" shown above).