Spectroscopy originally referred to the use of visible light dispersed according to its wavelength by a prism. Now-a-days the concept refers to the study of the interaction between matter (viz., atoms, molecules, ions, and solids) and electromagnetic radiation as a function of the radiation wavelength. When light passes through a medium, some of the light energy may be absorbed, in addition to other processes such as scattering, surface reflectance, etc. If there is a fractional absorption of energy, then the medium is called a transparent medium (to that particular radiation). When a portion of the radiation is absorbed from a passing continuous radiation through a transparent substance, the absorbed wavelengths are found absent the residual radiation spectrum. This is known as an absorption spectrum. UV-Visible absorption spectroscopy (along with emission and scattering spectroscopy) provides useful qualitative and quantitative information regarding matter and many physical and chemical processes.
A plot of the response as a function of wavelength - or more commonly frequency - is referred to as a spectrum.
When electromagnetic radiation passes through matter, most of the radiation continues in its original direction but a small fraction is scattered in other directions. Light that is scattered at the same wavelength as the incoming light is called Rayleigh scattering. Light that is scattered in transparent solids due to vibrations (phonons) is called Brillouin scattering. Brillouin scattering is typically shifted by 0.1 to 1 cm−1 from the incident light. Light that is scattered due to vibrations in molecules or optical phonons in solids is called Raman scattering. Raman scattered light is shifted by as much as 4000 cm−1 from the incident light.
When atoms or molecules absorb light, the incoming energy excites a quantized structure to a higher energy level. The type of excitation depends on the wavelength of the light. Electrons are promoted to higher orbitals by ultraviolet or visible light, vibrations are excited by infrared light, and rotations are excited by microwaves.