The Electromagnetic Spectrum

Electromagnetic radiation is commonly divided into 7 classifications as presented in the following figure. The figure also shows the amount of solar UV and visible light reaching the earth as a function of wavelength.



The shortest wavelengths in sunlight are the ultraviolet, extending down to a wavelength of about 295 nanometers. The earth's atmosphere filters out the shortest wavelength radiation, which is most of the ultraviolet region. But even though UV constitutes just 5 percent of the total sunlight reaching the earth's surface, these remaining short wavelengths are responsible for virtually all the damage to polymers.

The Quantum theory states that, unlike other forms of energy, light comes in discrete particles called photons. Photons behave much like tiny projectiles traveling at the speed of light. The "size" of a photon is inversely proportional to wavelength - the shorter the wavelength, the bigger the photon.

Photochemical reactions are caused by a single photon colliding with a single electron. If the photon is big enough (i.e., contains more energy than the organic bond strength) the electron is knocked out of its orbit and a reaction occurs. If the photon is not big enough, the energy of collision is harmlessly dissipated and no reaction occurs. For every chemical bond, there is a critical threshold size of photon (and hence a threshold wavelength) with enough energy to cause a reaction. Light of any wavelength shorter than the threshold wavelength can break the bond, but light of wavelengths longer can never break the bond.

An analogy to the Quantum Effect is throwing objects at a window. If you throw 5 lbs. Of sand at a window, one grain of sand at a time, the glass would remain intact. But if you threw a 5-lb. rock, the glass would shatter.