82 ELECTROMAGNETIC RADIATIONS AND MATTER 



The molecular extinction coefficient is strongly dependent upon wave- 

 length, as we shall soon see. The optical transmission is defined as 100 7// per 

 cent. The optical density, often used, is defined as log (I /I), and increases 

 linearly as concentration of absorber is increased. 



SOME INTERACTIONS OF ELECTROMAGNETIC RADIATIONS 



AND LIVING MATTER 



The parts of the spectrum which are of biophysical importance can be 

 conveniently classified under four main titles: the warming region, the visible 

 region, the photochemical region, and the ionizing region. Each of these is illus- 

 trated below. Enough of the principles are given to introduce infrared and 

 ultraviolet therapy. The visible region is considered in more detail, for 

 obvious reasons. X and gamma rays, and hard ultraviolet too, are intro- 

 duced here in principle only. Detection and absorption are discussed in 

 Chapter 5, and Chapter 9 deals exclusively with biological effects of all the 

 ionizing radiations. 



The Warming Radiations (Infrared) 



Electromagnetic radiation in the infrared range is always associated with 

 heat energy of those molecules which contain permanent dipoles. Its ab- 

 sorption results in increased rotations and vibrations, and therefore in in- 

 creased temperature. Infrared radiations are then logically called "heat 

 rays. 



The penetration into tissue is appreciable, although the extinction coeffi- 

 cient is large. The warming effect of absorption by the very outer layers of 

 the skin can be felt beneath the surface because of the poor but substantial 

 heat conduction of the tissue. Infrared-lamp therapy is based on this prin- 

 ciple. Since the tissue is 85 per cent water, the strongest absorption would 

 be expected to occur particularly near the strong water-absorption wave- 

 lengths: (1) vibrations at 28,200 and 63,000 A, (2) rotations from 500,000 

 to 1,200,000 A, as well as (3) some absorption by mixed vibrations and ro- 

 tations at nearly all wavelengths greater than about 8000 A. Intense infra- 

 red electromagnetic radiation, when absorbed by tissue, causes gas and 

 steam pockets which lead to lesions and blisters. 



Infrared Spectra 



The wavelengths absorbed often provide clues as to what rotation or 

 vibration is absorbing the incoming radiation. In the instrument called the 

 spectrometer a small slit of light from a continuously burning carbon arc — 

 a good source of infrared radiation — passes through the absorbent and then 

 on through a triangularly shaped crystal (prism) of KC1 or KBr; the trans- 

 mitted radiation is broken up — the longer wavelengths will be bent sharply 



