ELECTROMAGNETIC RADIATION; NATURE AND SPECTRUM 77 



tion is simply a question of providing a medium which can absorb the light, 

 or a medium with which the light can interact and be partially absorbed, to 

 appear as another, more familiar form of energy. 



Electromagnetic radiation propagates with undiminished energy through 

 a vacuum, always at the speed of light no matter what the frequency. 



The Electromagnetic Spectrum — A Survey 



Table 4-1 gives some properties of interest for the whole spectrum of elec- 

 tromagnetic radiations. Since the em radiation has both wave and particle 

 properties, the wavelength range of the different sections is given, and the 

 energy associated with an excitation in each section is given in electron volts 

 (1 electron volt/molecule = 22,000 cal/mole). Common means of detecting 

 and of handling the radiations are noted; and what happens during absorp- 

 tion is indicated. 



If one expects to gain insight into the interactions of electromagnetic 

 radiations and matter, one must study the two Tables, 4-1 and 4-2, ex- 

 haustively. There is no easier way. One will find, for example, from in- 

 spection of the dimensions of the wavelength, A, and frequency, v, that 

 they are related through the velocity, c, which for all electromagnetic radia- 

 tions in vacuum, no matter what the wave length, is 3 x 10 10 cm/sec 

 (186,000 miles/sec). Thus 



v = 3 x 10 !0 /A cycles/sec 



Table 4-2 indicates some of the effects of the interaction of various "cuts" 

 of the spectrum with matter. It is certainly true that radiation of short wave 

 length (high frequency) carries more energy, is more penetrating, and can 

 do more damage than that of long wave length. Thus, at wavelengths from 

 20,000 to 500,000 A, the radiation simply tickles the molecules into a rota- 

 tional and vibrational frenzy (high heat energy;. Radiation of 4000 to 

 7800 A excites electrons in the pigment molecules of the retina of the eye, 

 and is visible. (Maximum sensitivity of the eye is at about 6000 A.) Radia- 

 tion of wavelength 2000 to 4000 A (ultraviolet) excites even the bonding elec- 

 trons in a molecule, and so loosens up a bond that chemical reactions may 

 take place which otherwise could not. Wavelengths below 2000 A, in the 

 hard or vacuum ultraviolet, actually drive electrons out of a molecule, or 

 ionize it; and as the wavelength gets shorter, and the radiation "harder," 

 more and more ions are formed in the wake of the incoming radiation. In 

 the X-ray region (X = 1 A) the electrons of even the K shell of the atom, 

 the most tightly bound ones, can be excited or ejected; and in the gamma 

 region (~0.01 A), even the nucleus can be penetrated by the radiation, al- 

 though electrons in the atomic cloud are a more probable target. 



