A spectrum is a representation of what electromagnetic radiation is absorbed or emitted by a sample. The representation could be a plot, a diagram on a computer screen, even a list of wavelengths and intensities.
The word "spectrum" was coined by Isaac Newton, from a Latin word meaning "appearance." (The word "spectre," meaning ghost, shares the same root.) Newton projected his spectrum (plural spectra) on a wall or screen, and for a few hundred years the projection of a spectrum onto a surface was the best that could be done. (And even under such conditions, progress was made!) With the development of photography and, ultimately, electrical and electronic detection devices, a spectrum could be recorded permanently. Ultimately, the word "spectrum" came to represent the permanent record, rather than the dispersed white light (or other region of the electromagnetic spectrum).
In this chapter, we will treat a spectrum as the physical record of which wavelengths/frequencies/energies of light are absorbed or emitted by a sample. With that in mind, we will find that there are several popular ways of displaying a spectrum, although different types of spectroscopy typically display spectra in one or two common ways.
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