Contemporary Advances in Physics, XVII 

 The Scattering of Light with Change of Frequency 



By KARL K. DARROW 



SCATTERING of light is one of the commonest of all phenomena, 

 which does not in the least imply that it is one of the most com- 

 monplace. Even its practical importance entitles it to high respect. 

 We are often told that were it not for scattering, the sky would not be 

 blue; the sun and the stars would stand out amazingly brilliant against 

 a background black as coal. It is probable, however, that if scattering 

 were suddenly to be suspended, the disappearance of the sky would be 

 one of the least of our worries. Everything else would disappear, 

 except what was self-luminous. The visible world would consist of 

 the sun, the other stars, and flames, some electrical discharges, the 

 filaments of incandescent lamps, and some substances glowing feebly 

 with fluorescence or phosphorescence. Nothing else could be seen 

 except as a silhouette, apart from objects so translucent that they could 

 be viewed as a stereopticon slide against a flame. Happily no such 

 calamity impends; and we may unconcernedly consider the theoretical 

 importance of the process, which is great. As some might say, the 

 scattering of light is one of the battlegrounds between the undulatory 

 and the corpuscular theories. Metaphors of combat are however not 

 appropriate; it is necessary to reconcile the theories, not to smash 

 one or the other. Now it happens that some of the phenomena of 

 scattering may be interpreted by the one theory, and some by the 

 other; and some can be explained by either, which is most auspicious; 

 for if this can some day be said of all the phenomena of light, the goal 

 of our desires will have been attained. 



Also scattering of light has just sprung into prominence as the most 

 inviting and the most ardently invaded field of physics, because of a 

 discovery such as, it had been supposed, could never happen again. 

 It seemed that experimental physics had been so thoroughly developed 

 — or, to change over to an ancient metaphor, that the field had been 

 so thoroughly harvested and then so exhaustively gleaned — that 

 nothing important could possibly remain to be discovered unless by 

 measurements of great precision, or by radically new apparatus, or by 

 applying voltages or other agencies on a scale as yet untried. Yet in 

 the spring of 1928 a mode of scattering of visible light was discovered, 



64 



