280 



TEMPERATURE AND HUMIDITY 



flow of heat by radiation does not depend 

 upon the presence of an intervening me- 

 dium, and heat wll pass by the process of 

 radiation from a hot object to a cooler one 

 through a vacuum. Fig. 2 shows diagram- 

 matically the relative positions of some of 

 the various parts of the spectrum, and, as 

 examples, the energy distribution of the 



T constant) it can be shown that the wave- 

 length of the maximum radiation is 



(3) X max. T = constant 



= 0.288 cm. degrees. 



This formula is known as "Wien's displace- 

 ment law" because, as the temperature of 

 the radiating object increases, the wave- 



UJ 



\- 

 z 



5 



viSi-; 



BLEI 



NEAR INFRARED 



WAVE LENGTH 

 Fig. 2. Relative position in the spectrum of radiation from various sources 



FAR 

 INFRA- 

 RED 



RADIO 

 WAVES 



radiation from the sun, a red-hot stove, and 

 the human skin, as given by Blum (6). 



Laws of Radiation 



p The amount of energy which is radiated 

 at wavelength X, and included in the small 

 spectral range, X, is given by the law of 

 Planckffor black body radiation: 



CiX-5 

 (2) 



//x = 



C2/xr 



- r 



where Ci = 4.93 X 10~i^ (X expressed in 

 cm.), Ci = 1.432 cm. degrees, and T = 

 absolute temperature. If X is large, then 

 the minus unity in the denominator may be 

 neglected and we have: 



which is Wien's Law . By simple differentia- 

 tion of Wien's Law with respect to X (holding 



length of the maximum radiation is dis- 

 placed towards the shorter wavelengths. 

 Thus, in Fig. 2, the sun's radiation (T = 

 6000° K) has its maximum at about 0.5 n, 

 whereas the hot stove {T = 1000° K) has its 

 maximum at about 3 m and the human body 

 {T = 300° K) radiates maximally at 9.5 m- 

 Consideration of Wien's displacement law 

 (together with the eye's visibility curve) 

 shows why one should expect the sun's 

 radiation to appear white, that of the stove 

 red, and that of the human skin to be 

 invisible. 



Adding up the values of H\ for all wave- 

 lengths gives the Stefan Boltzman Law for 

 total radiation : 



(4) 



Ha = So T\ 



The net transfer of heat is the difference in 

 the radiation emitted and the radiation 



