A nalytical chemists frequently use the measurement of light absorption to determine concentration of chemicals. 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. Energy is absorbed by the C-H bond at a specific wavelength of light when the energy difference between v 1 and v 2 equals the energy of the light. 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.