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Chapter 4:
The Spectrum
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 "€œspectrum" was coined by Isaac Newton, from a Latin word meaning "€œappearance."€ (The word "€œspectre," 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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