CONTEMPORARY ADVANCES IN PHYSICS 83 



infra-red spectrum of the scatterer has not yet been explored; and 

 there we may deduce its lines from the frequency-shifts of the Raman 

 lines in the visible spectrimi. Moreover, there are regions of the infra- 

 red spectrum which are very difficult to explore, because of such tech- 

 nical reasons as the insensitiveness of photographic plates; and the 

 Raman lines make it possible to discover some at least of the features 

 of these, by observations made in the most convenient region of the 

 spectrum. Perhaps this will turn out to be the most fruitful of the 

 consequences of Raman's discovery. 



Scattering of Light with Transfer of Energy to 

 Vibrations in Solids 



The scattering of light with shift of frequency from solids was 

 discovered, independently and almost simultaneously, by C. V. Raman 

 and K. S. Krishnan in India and by G. Landsberg and L. Mandelstam 

 in Russia. As seen on the photographs of the spectra of the scattered 

 light, the effect is altogether like the Raman effect of liquids and 

 vapors. The lines of the primary spectrum, scattered without change 

 of frequency, are accompanied by companions shifted mostly towards 

 lower, but in occasional cases towards higher frequencies. The in- 

 falling quanta therefore sometimes cede energy to quantized motions 

 within the solid substance, and sometimes — but much less frequently — - 

 receive energy from these. 



The first and obvious question is: do the frequency-shifts agree with 

 lines of the infra-red spectra of the solid substance? For studying the 

 infra-red spectra of solids there are, be it remembered, two classical 

 methods. One is the familiar way of dispersing a beam of light which 

 has traversed the solid, and looking for absorption lines or bands in its 

 spectrum. To find a good dispersing-agent in the far infra-red is 

 however not easy; and there is an alternative method, in which the 

 beam of light is reflected several times over from samples of the solid. 

 At each incidence of the beam upon the crystal, the waves of frequen- 

 cies which do not coincide with natural frequencies of the substance go 

 on through, while the waves of frequencies which do coincide are mostly 

 reflected. Thus, after several reflections, the "residual" beam is 

 composed of one or a few wave-lengths, those of the principal absorp- 

 tion-lines of the crystal ; and these are measured by operating on the 

 beam with a special interferometer, or in some other way. This is the 

 method of "residual rays," or "Reststrahlen," which was developed 

 and much exploited during the nineties of the last century and the 

 opening years of this. The spectrum-lines of the crystalline substance 

 are its "Reststrahlen"; and these are to be compared with the fre- 



