Contemporary advances in physics, XVII the scattering of light with change of frequency

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 commonplace. 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.