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Spectrophotometry: The Absorption of Light
Analytical chemists frequently use the measurement of light absorption to determine concentration of chemicals. The technique is called spectrophotometry or spectroscopy, but why is light absorbed? Light may be simply scattered by particles, but this is detrimental to the measurement of truly absorbed light. Light is energy, and when energy is absorbed by a chemical it results in a change in energy levels of the chemical. Molecules normally exist in discrete energy levels. Vibrational energy levels exist because molecular bonds vibrate at specific frequencies. Electronic energy levels exist because electrons in molecules can be excited to discrete, higher energy orbitals.
The energy (E) of light depends on its wavelength. Longer wavelengths (infrared) have less energy than shorter wavelengths (ultraviolet). A molecule will absorb energy (light) when the energy (or wavelength) exactly matches the energy difference between the two energy states of the molecule.
E = h v = h c / l
where v = frequency of light, l = wavelength of light, h = Planck's constant, and c = speed of light
For example, a carbon hydrogen bond (C-H) vibrates at frequency v1 and a higher frequency v2. Energy is absorbed by the C-H bond at a specific wavelength of light when the energy difference between v1 and v2 equals the energy of the light.
E = h ( v1 - v2 )
Infrared light contains enough energy to cause changes in vibrational energy levels, and this forms the basis of Infrared Spectroscopy. UV and visible light contains enough energy to excite electrons in molecules, and forms the basis for UV-VIS Spectroscopy.
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