Orginally a spectrum was what was observed when white light was dispersed through a prism. Later the term referred to a plot light intensity as a function of frequency or wavelength. Planck later realized that this represents electromagnetic energy.
E = h ν
Today the term has been generalized even further to include any data that are the result of a study where an energy 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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