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Matter can capture electromagnetic radiation and convert the energy of a photon to internal energy. This process is called absorption. Energy is transferred from the radiation field to the absorbing species. We describe the energy change of the absorber as a transition or an excitation from a lower energy level to a higher energy level. Since the energy levels of matter are quantized, only light of energy that can cause transitions from one level to another will be absorbed.
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.
Absorption spectroscopy is one way to study the energy levels of the atoms, molecules, and solids. An absorption spectrum is the absorption of light as a function of wavelength. The spectrum of an atom or molecule depends on its energy-level structure, making absorption spectra useful for identifying compounds.
Measuring the concentration of an absorbing species in a sample is accomplished by applying the Beer-Lambert Law.
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