36 



INFRA-RED REFLECTION SPECTRA. 



Drude's, also Schuster's Optics), the absorption coefficient is so low that 

 it is negligible, and the reflecting power is a function of only the refractive 

 index. Here the reflecting power is low, only 4 to 6 per cent, and de- 

 creases with increase in wave-length. All transparent media thus far 

 examined (except silver chloride) have bands of selective reflection. In 

 these bands the absorption coefficient k attains high values. 



If k becomes sufficiently large (see Schuster's Optics, Pockel's Crys- 

 tallographie, 1906), of the order unity, the absorption affects the reflecting 

 power, and the heat and light waves no longer enter the substance, but are 

 almost totally reflected, as in metals, whence the name, "bands of metallic 

 reflection." For metals, "electrical conductors," the absorption coefficient 



2 3 4 5 6 7 8 9 10 II 12 

 Fig. 24. Reflecting power; gold, silver, quartz, carborundum. 



I4-/1 



is so large that nearly all the energy for all (gold and silver are exceptions) 

 wave-lengths is reflected. In other words, the reflecting power of plane 

 surfaces ("regular reflection") of "transparent media" (electrical non- 

 conductors) will be low in all regions of the spectrum, except where there 

 are bands of "metallic reflection." It is evident that the diffuse reflection 

 from rough surfaces of transparent media must also be selectively reflecting. 

 For electrical conductors the reflecting power is high throughout the infra- 

 red spectrum. The great dissimilarity in the reflecting power of these two 

 classes of substances is well illustrated in fig. 24, which contains the graphs 

 of the reflecting power of gold, silver, quartz, and carborundum. 



Apropos of the illustration just quoted, of the reflection from smoked 

 glass, Very 1 "records that the percentage of reflected rays, as measured by 



1 Very: Astrophys. Jour., 8, p. 278, 1898. 



