Orginally a spectrum was what was observed when white light was dispersed through a prism. Soon the term referred to a plot of light intensity as a function of frequency or wavelength. Planck later realized that frequency represents electromagnetic energy.
Today the term has been generalized even further to include any data that are the result of a study where an energy (or frequency) is systematically varied
The recording and studying of spectrum of energy levels in atoms or molecules in the physical sciences[?] is called spectroscopy. A device for recording a spectrum is a spectrometer or spectrophotometer. The latter term is used when an optical spectrum is recorded by the device.
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wikipedia.org dumped 2003-03-17 with terodump