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Spectroscopy

Dispersion of light as it travels through a triangular prism

Spectroscopy was originally the study of the interaction between radiation and matter as a function of wavelength ( λ ). In fact, historically, spectroscopy referred to the use of visible light dispersed according to its wavelength, e.g. by a prism. Later the concept was expanded greatly to comprise any measurement of a quantity as a function of either wavelength or frequency. Thus it also can refer to a response to an alternating field or varying frequency ( ν ). A further extension of the scope of the definition added energy ( E ) as a variable, once the very close relationship

E = hν for photons was realized ( h is the Planck constant). A plot of the response as a function of wavelength—or more commonly

frequency—is referred to as a spectrum; see also spectral line width.

Spectrometry is the spectroscopic technique used to assess the concentration or amount of a given chemical (atomic, molecular, or ionic) species. In this case, the instrument that performs such measurements is a spectrometer, spectrophotometer, or spectrograph.

Spectroscopy/spectrometry is often used in physical and analytical chemistry for the identification of substances through the spectrum emitted from or absorbed by them.

Spectroscopy/spectrometry is also heavily used in astronomy and remote sensing. Most large telescopes have spectrometers, which are used either to measure the chemical composition and physical properties of astronomical objects or to measure their velocities from the Doppler shift of their spectral lines.

Monochromatic

In physics, the word "monochromatic" is used to refer to electromagnetic radiation of a single wavelength. In the physical sense, no real source of electromagnetic radiation is purely monochromatic, since that would require a wave of infinite duration as a consequence of the Fourier transform's localization property (cf. spectral coherence). Even sources such as lasers have some narrow range of wavelengths (known as the spectral linewidth) within which they operate. In practice, filtered light, diffraction grating separated light and laser light are all routinely referred to as monochromatic. Often light sources can be compared and one be labeled as “more monochromatic” (in a similar usage as monodispersity ). And a device which isolates light sources of a narrow bandwidth are called monochromators, even though the