Spectroscopy:

 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 linewidth.

 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.

The type of spectroscopy depends on the physical quantity measured. Normally, the quantity that is measured is an intensity, either of energy absorbed or produced. 

• Electromagnetic spectroscopy involves interactions of matter with electromagnetic radiation, such as light.
• Electron spectroscopy involves interactions with electron beams. Auger spectroscopy involves inducing the Auger effect with an electron beam. In this case the measurement typically involves the kinetic energy of the electron as variable.
• Acoustic spectroscopy involves the frequency of sound.
• Dielectric spectroscopy involves the frequency of an external electrical field
• Mechanical spectroscopy involves the frequency of an external mechanical stress, e.g. a torsion applied to a piece of material.