On the molecular scattering of light in water and the colour of the sea

Abstract

The theory that the light of the sky owes its origin and colour to diffraction by the molecules of the atmosphere is now established on a firm experimental basis by the brilliant work of Cabannes and of Lord Rayleigh on the scattering of light by dust-free gases, and by the measurements of the transparency of the higher levels of the atmosphere in the visual region of the spectrum, which have yielded results in close agreement with the calculated values. It is the purpose of the present paper to point out the part played by molecular diffraction in another of the great natural optical phenomena, that is, the colour exhibited by large masses of clear water when illumined by sunshine and viewed from above, the depth being so great as to provide a perfectly black background for observation. The subject at present is in a somewhat peculiar position, as even on the question of the reality of the phenomenon there appears to be a divergence of opinion. On the one hand, we have the following view expressed by so eminent an authority as the late Lord Rayleigh: “We must bear in mind that the absorption, or proper, colour of water cannot manifest itself unless the light traverses a sufficient depth before reaching the eye. In the ocean the depth is, of course, adequate to develop the colour, but if the water is clear, there is often nothing to send the light back to the observer. Under these circumstances, the proper colour cannot be seen. The much-admired dark blue of the deep sea has nothing to do with the colour of water, but is simply the blue of the sky seen by reflection.” On the other hand, observers familiar with the sea, such as J. Y. Buchanan, of the “Challenger” expedition, who have had very wide opportunities for study, have published detailed descriptions which support an entirely contrary view. An admirable précis of the literature on the whole subject has been recently published by Prof. W. D. Bancroft. From a perusal of this very convenient summary, and from the account given in Kayser and Bunge’s ‘Handbuch,’ it would appear that the general trend of opinion is that, so far as there is any real effect to be explained at all (that is, apart from reflected sky light) the colour of water is due to absorption, the return of the light from the depths of the liquid being due to suspended matter in it. During a recent ocean voyage, the present author has had an opportunity of making some observations which show that the view indicated above is entirely inadequate. It is proposed in this paper to urge an entirely different view, that in this phenomenon, as in the parallel case of the colour of the sky, molecular diffraction determines the observed luminosity and in great measure also its colour. As a necessary preliminary to the discussion, a theoretical calculation and experimental observations of the intensity of molecular scattering in water will be presented.