Chapter 12: Electromagnetic Waves (545 KB) - D Cassidy Books

3637_CassidyTX_12 6/13/02 11:34 AM Page 576576 12. ELECTROMAGNETIC WAVES2000. Nevertheless, the chlorine already in the atmosphere can continuethe destruction of the ozone layer for several decades. The result has beena steady increase in harmful effects of ultraviolet radiation. These effectsare predicted to continue for decades to come until most of the chlorinepreviously emitted into the atmosphere is finally rendered harmless. In themeantime, physicians recommend that in open sunlight everyone wear sunglassesthat actually filter out ultraviolet rays (in all forms), and not to stayin the Sun without wearing a hat and applying sunblock lotions to exposedskin.X rays ( 10 8 m to 10 17 m; f 10 16 Hz to 10 25 Hz). Atoms emit Xradiation when electrons undergo transitions between the inner shells ofthe atoms. X rays are also produced by the sudden deflection or stoppingof electrons when they strike a metal target. The maximum frequency ofthe radiation generated is determined by the energy with which the electronsstrike the target. In turn, this energy is determined by the voltagethrough which the electrons are accelerated. So the maximum frequencyincreases with the accelerating voltage. The higher the frequency of theX rays, the greater is their power to penetrate matter. But the distance ofpenetration also depends on the nature of the material being penetrated.X rays are readily absorbed by bone, which contains calcium, while theypass much more easily through less dense organic matter such as flesh,which contains mainly the light atoms: hydrogen, carbon, and oxygen.These properties of X rays, combined with their ability to affect a photographicplate, have led to some of the spectacular medical uses of X-rayFIGURE 12.19 X-ray photographof chambered nautilus sea shell.