<unk>  was originally the study of  the interaction between radiation and matter as a function of wavelength λ. In fact, historically,<unk>  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 function of either wavelength or frequency. Thus it also can refer to interactions with particle radiation or to a response to an alternating field or varying frequency ν. An 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. A plot of the response as a function of wavelength — or more commonly frequency — is referred to as a spectrum.
Spectrometry is the measurement of these responses and an instrument which performs such measurements is a spectrometer or spectrograph, although these terms are more limited in use to the orginal field of optics from which the concept sprang.<unk>  is often used in physical and analytical chemistry for the identification of substances through the spectrum emitted from or absorbed by them.<unk>  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.
Spectroscopy
Visible spectrum
Chemiluminescence
Electromagnetic spectrum
Absorption spectrum
Neurochemistry
Ion chromatography
Clinical chemistry