Papers ill Marine Biology and Oceanography, Suppl. to vol. } of DeepSea Research, pp. 426-434. 



Polarization of scattered sunlight in deep water* 



By Talbot H. Waterman 



Osborn Zoological Laboratory, Yale University, New Haven, Conn. 



Summary — 1. Photographic measurements of the polarization pattern of underwater illumination 

 were made at depths of 30, 60, 100, 150, and 200 m in oceanic water with a vertical extinction coefficient 

 corresponding to Secchi disc readings between 22-26 m. A special polarization analyzer, attached to 

 the outside of a deep-sea camera case, provided in a single photograph direct evidence for: (a) 

 presence or absence of polarization with a high degree of sensitivity; (b) type of polarization; (c) 

 plane of polarization, if linear; (d) rough comparative indication of the degree of polarization. By 

 photographing the interference pattern formed by this analyzer when traversed by polarized light, 

 records of the polarization of horizontally scattered sunlight were obtained in four azimuths relative 

 to the sun's bearing: 0°, 90°, 180\ 270°, at eleven different stations, including two at each depth 

 except 200 m. 



2. Horizontally scattered light was found to be polarized at all depths, 200 m included, and in all 

 directions tested; the type of polarization was linear in every instance. In so far as could be judged 

 from the photographs (Fig. 3), the percentage polarization at 200 m appeared to be at least as great 

 as at lesser depths. 



3. The plane of polarization was always horizontal in lines of sight toward and away from the sun's 

 bearing but was tilted toward the sun in azimuth's normal to these; this effect of the sun's position 

 was still considerable at 200 m. The amount of tilting occurring in lines of sight 90° from the sun's 

 bearing was primarily related to the angle of refraction for sunlight at the air-water interface (Fig. 2), 

 but a second effect, resulting from depth of light penetration, was shown by the decrease in the tilting 

 of the polarization plane as depth was increased. 



4. [t is concluded that: (a) scattered sunlight underwater is linearly polarized to considerable 

 depths, probably throughout the photic zone; (b) the pattern of polarization is dependent on the 

 sun's position at least to a depth of 200 m and (c) such optical phenomena therefore may be important 

 in the behaviour of photic zone bathypelagic animals as well as epipelagic forms. 



INTRODUCTION 



Previous measurements (Waterman, 1954 a) have shown that natural underwater 

 illumination is linearly polarized in patterns which seem basically similar to those of 

 the blue sky.j This is true of the upper 15 m or so of water to which the earlier work 

 was limited. Since the most probable origin for this polarization would appear to be 

 the molecular or fine particle scattering of directional light rays penetrating the water, 

 it was predicted that linear polarization of submarine illumination should occur 

 as deep as significant amounts of directional light are transmitted. It was also 

 indicated that, since certain terrestrial and shallow water animals are known to be 

 capable of utilizing polarized light in the sky for their orientation and navigation, 

 underwater polarization may prove to be a factor of considerable ecological importance 



* Contribution No. 763, Woods Hole Oceanographic Institution. 



t This introductory statement refers only to polarization arising within the water itself. It does not 

 include the direct underwater observation or measurement of polarized sky light. The known relation 

 between these two sources of polarization has previously been described for shallow water (Water- 

 man, 1954 a). Further measurements will be required to establish in deeper water the effective 

 penetration and influence of polarized sky light, as opposed to direct sunlight, which is unpolarized 

 in the atmosphere. 



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