76 ELECTROMAGNETIC RADIATIONS AND MATTER 



ELECTROMAGNETIC RADIATION; NATURE AND SPECTRUM 



The electron clouds of atoms and molecules can be excited by various 

 methods — by heat, bombardment by some charged particle, and by absorp- 

 tion of incoming radiations. A simple example is the flame test for sodium: 

 if a sodium salt is heated in a flame, it glows with a characteristic yellow 

 glow. It is not burning (i.e., being oxidized by oxygen). Rather, the valence 

 (outermost) electron gets excited (accepts energy) and "jumps" to a higher- 

 energy orbital, from a 3s to a 3/?. Imagine the next set of orbitals around 

 the nucleus in Figure 4-3. Its lifetime there is short, however, and it falls 

 back to the original state ("ground state"), and emits the extra energy as 

 electromagnetic radiation {light in this case) of such a wavelength (5893 A) 

 that it excites the cone cells on the retina of the eye. 



Biology is entering its electromagnetic age. Many parts of the electromag- 

 netic spectrum are beginning to be used for diagnosis and therapy, as well 

 as for studies which are leading to a better understanding of the roles of 

 each of the parts in the systematized whole. 



Nature of Electromagnetic Radiation 



The exact nature of electromagnetic (em) radiation is unknown. What is 

 known is that the wave has two component parts, an electric part and a mag- 

 netic part, moving in phase, but in direction 90° from each other — much like 

 two vibrating strings, one going up and down while the other goes back and 

 forth — superimposed on each other. Each oscillates about an average value 

 (zero) at a frequency which depends upon electronic vibrations in the 

 source. The em waves travel in a straight line, and have energies inversely 

 proportional to the wavelength, or directly proportional to the frequency 

 (number of cycles per second). The wave carries no net electrical charge, 

 and no net magnetic moment, but because of the components which can in- 

 terfere or react with electric or magnetic fields, it can lose or gain energy 

 (i.e., change frequency). All em waves travel at the velocity of "light." 

 They have both wave properties (such as the capability of being reflected or 

 diffracted) and particle properties (such as delivering their energy in 

 bundles or quanta.). The unit bundle of electromagnetic energy is called 

 the photon. Undulations in the electromagnetic field are described by the 

 celebrated Maxwell equations (1873). 



Electromagnetic radiations vary only in frequency, and through this, in 

 energy. Therefore their use requires handling the energy contained in the 

 radiation. For example, we know how to handle light with mirrors, lenses, 

 microscopes, and prisms, and to detect it by photographic plates, photo- 

 electric cells, the eye, etc. Handling, or making it serve a useful purpose, is 

 simply a question of using equipment which does not absorb the light. Detec- 



