PAPERS ON CHEMISTRY AND PHYSICS 267 



THE PROBLEM OF COLD LIGHT 



Harvey A. Neville, University of Illinois 



The exact nature of light is not understood, but our 

 present conception of it is something of a compromise 

 between the wave theory and the earlier corpuscular 

 theory. From a consideration of the recent discoveries 

 of Planck, Thompson and Einstein it is concluded that 

 light consists of discrete particles or atoms of energy, 

 each with a specific energy content E=h\/, moving with 

 velocity c, and having a mass e/c 2 . (h is Planck's con- 

 stant =6.55xl0~ 27 ergs per sec. ;V is the frequency of 

 vibration.) 



Light energy is emitted by matter when in an excited 

 state. This excitation may be induced by various means, 

 the most general being by increasing the temperature of 

 the system. A black body is non-luminous up to 500° C. 

 due to the limit of sensitivity of the human eye. If the 

 eye were sensitive to longer wave lengths of the infra- 

 red, lamp-black would appear highly luminescent among 

 other bodies at the same temperature. All substances 

 whose temperature is above absolute zero emit simple 

 thermal radiation. The higher the temperature the more 

 rapid the vibration of the atoms and electrons and there- 

 fore the shorter the wave length. At sufficiently high 

 temperature the wave length of the emitted radiation 

 corresponds to the range of the visible spectrum. 



The total radiation of a body increases directly at the 

 fourth power of the absolute temperature; as stated by 

 Stefan's law R=kT*. As the temperature rises the 

 maximum-power radiation recedes to shorter wave 

 lengths. The expression of this relation, known as 

 Wien's law, is 

 Ac^vT- 1 or AT=k. 



A body which is emitting liglit by purely thermal radia- 

 tion, in agreement with the two laws just mentioned, is 

 said to be hi candescent. A body which is emitting a 

 greater total radiation than can be accounted for by its 

 temperature alone is said to be luminescent. It is light 

 produced in this way, unaccompanied by the theoretical 

 amount of heat, that is referred to as cold light. For 



