Energy, Frequency, Wavelength and the Electromagnetic Spectrum

To understand photochemistry at the molecular level, we must first consider electromagnetic radiation in general. The energy of electromagnetic radiation is described by the familiar equations

E = hn = hc/l

E = energy of the radiation
h = Planck’s constant
n = frequency of the radiation
l = wavelength

Note from these equations that energy is directly proportional to frequency and inversely proportional to wavelength, so that shorter wavelength, higher frequency radiation is of higher energy, and longer wavelength, lower frequency radiation is of lower energy.

When electromagnetic radiation strikes matter, the radiation can either be reflected, transmitted, or absorbed. If the radiation is absorbed, the energy of the molecules in the matter is increased and the molecules are then said to be in an excited or higher energy state. Depending upon the energy (or frequency or wavelength) of the incident radiation, different molecular processes can occur.

Absorption of electromagnetic radiation by molecules is quantized - that is, the molecules absorb radiation of specific frequencies only, depending upon the exact structures of the molecules. In order of decreasing energy of radiation, the electromagnetic (EM) spectrum consists of X-rays, ultraviolet, visible, infrared, microwave, and radio waves. Radiation in each portion of the EM spectrum interacts with matter differently, causing different molecular processes to occur. Each will be briefly described starting at the high energy end of the spectrum.

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