A molecule may absorb electromagnetic (em) radiation and, in the process, break down into its atomic or molecular components. Unstable atoms and molecular fragments may also combine to form more stable molecules, disposing of their excess energy in the form of em radiation. These chemical reactions are called photochemical, and the process by which a photochemical reaction occurs is called photolysis. Photochemical reactions play very important roles in many aspects of environmental chemistry. Therefore, this book concludes with a brief account of some of the basic principles of photochemistry, which we will then apply to ozone in the Earth's stratosphere and the problem of the stratospheric ozone hole.

Some properties of electromagnetic waves

Electromagnetic radiation has both wave and particle characteristics. Considered as a wave, em radiation may be viewed as an ensemble of waves that travels through a vacuum with the speed of light, c = 2.998 x 108ms-1. The distance between two successive crests in the intensity of the radiation is called the wavelength (λ) of the radiation. The frequency (v) of the radiation is the number of crests in intensity that pass a given point in one second. The units of v are s-1 or hertz (Hz). If v crests in intensity pass a given point in one second, one crest passes a given point in 1/v seconds. Therefore, a crest in intensity of an em wave travels a distance λ in 1/v seconds.