2 INTRODUCTION 



where o- is a constant (Stefan's constant) and where T is the absolute 

 temperature. This relation is known as Stefan's law. If radiation is 

 measured in gram calories per square centimeter per minute, the numeri- 

 cal value of 0- is 82 X 10"^^. The energy of radiation of different wave 

 lengths, X, is expressed by Planck's law, which can be written 



Y^ - ~ji jy^^Jy 



where E\ is the energy emitted per unit area and unit time within a unit 

 range of wave length and where ci and C2 are constants. The function 

 / (XT') is zero for X = and X = oo and is at a maximum when the value 

 of XT equals 2940. Therefore the wave length of maximum radiation, 



Fig. 1. Energy emitted by a black body at different wave lengths and different 

 absolute temperatures. 



X„», equals 2940/7; that is, for bodies of temperature 6000°, 300°, and 

 200° the maximum energy of the radiation is nearly at wave lengths 

 0.5 M, 10 M, and 15 M, respectively. 



The distribution of the energy can be found by plotting /(XT) against 

 XT (fig. 1). The relation between the emitted energy at a given tempera- 

 ture T and wave length X is found from this figure by dividing the scale 

 values along the abscissa by T and multiplying the scale values along the 

 ordinate by T^. 



Selective Radiation and Absorption. Most gases and vapors do 

 not radiate as black bodies, but at certain wave lengths they emit 

 radiation of an intensity comparable to the intensity of black-body 

 radiation of the same temperature. If the rate at which a body radiates 

 from unit area is called ''emissive power" (Brunt, 1939), it can be stated 

 that the emissive power of a black body is a continuous function of 



