168 Microbes and You 



RADIATIONS 



VISIBLE SPECTRUM 



The portion of the Hght spectrum with wavelengths large 

 enough to be visible to the human eye is called the visible 

 spectrum. Wavelengths detectable only with the use of special 

 instruments are called ultraviolet at the blue end of the scale and 

 INFRA-RED at the red end of this band. These wavelengths are 

 measured in Angstrom units— one ten-millionth of a millimeter in 

 length. The eyes of man are able to see light with wavelengths 

 from 3900 Angstroms at the violet end to about 8000 Angstroms at 

 the red end of the band. Different eyes have different seeing 

 abilities, so these upper and lower limits are only approximations. 



In general, the visible spectrum has little adverse effect on 

 bacteria, but certain bands of invisible light are decidedly germi- 

 cidal. Most research has been directed at the ultaviolet rays, 

 which leaves a fertile field of research for persons interested in 

 studying the action of other bands of the light spectrum on 

 microbes. 



SUNLIGHT OR SOLAR RADIATION 



The destructive action of sunlight on bacteria was first pointed 

 out in 1877-1878 by Downes and Blunt. The lethal effect of sun- 

 light is attributed to the ultraviolet band, and its mode of action 

 appears to be a denaturation of the protein in bacteria. In the 

 absence of chlorophyll, which allows cells of higher plants to con- 

 vert simple materials into plant substance, bacteria cannot prevent 

 the accumulation of energy within the cells, and the organisms are 

 harmed by this type of light. Strong sunlight coming in direct 

 contact with bacteria exerts a destructive effect, just as we find 

 human skin adversely affected by prolonged contact with sunlight- 

 sunburn. Water purification and treatment of milk with ultra- 

 violet light have been advocated through the years, but the poor 

 penetrating power of ultraviolet light and the expense limit its 

 usefulness. 



The strongly bactericidal rays are those bet^veen 2500 and 2800 

 Angstrom units, and the effect of sunlight in helping to destroy 



