The Ability to See Clearly Underwater

Prasannasai k, M.Optom.

Optometrist, Binocular Vision Department, Sankara Nethralaya, Chennai, India

 

Underwater Vision The ability to see objects while submerged in water is known as underwater vision. It can be significantly impacted by various factors. For instance, the rapid decrease in light as it travels through water can result in lower levels of natural illumination, making objects less visible.⁽¹⁾

Refractive Index The refractive index of water is significantly different from that of air, which can impact the way the eye focuses.⁽²⁾

Sight Underwater ⁽³⁾

• Light Refraction: Light bends when it passes between two dissimilar media. The extent of bending is dictated by the refractive indices of the media.
• Human Eye Adaptation: The human eye is optimized for seeing in air Nonetheless, water has a refractive index comparable to that of the cornea, which nullifies its capacity to focus. When submerged, images are focused behind the retina instead of upon it, resulting in blurred sight due to hypermetropia

Moken Tribe 

The youngsters of a tribe in Thailand possess the singular capability to see distinctly underwater⁴. What facilitates their attainment of this capability? Is it feasible for others to acquire this proficiency?

Underwater Sight of Moken Youngsters The Moken tribe in Thailand is renowned for their youngsters’ capacity to see distinctly underwater. In 1999, investigator Anna Gislen conducted an experiment that revealed that Moken youngsters possessed twice the underwater visual acuteness of European youngsters who underwent the same experiment.⁽⁴⁾

Picture courtesy (https://ychef.files.bbci.co.uk/1600×900/p03l2g0p.webp)

Examination of Underwater Visual Sharpness ⁽⁴⁾

• Subjects: Six Moken youngsters and 28 European youngsters of the same age range
• Procedure: An apparatus to stabilize the head was established underwater at a fixed distance for viewing to display to the children achromatic sinusoidal gratings of varying spatial frequencies
• Findings: The Moken youngsters had significantly superior underwater sharpness and could distinguish gratings more than twice as fine as those distinguished by the European children

 Hypotheses on the Underwater Sight of Moken Youngsters ⁽⁵⁾

• It was postulated that the Moken youngsters either possessed an innate adaptation that modified the functioning of their eyes or had acquired the ability to utilize their eyes differently underwater
• An ocular examination demonstrated that the Moken youngsters had a comparable above-water sight to European youngsters of the same age range, rendering the first hypothesis improbable
• Sight underwater can be enhanced by modifying the form of the lens (accommodation) or by decreasing the dimensions of the pupil to augment the depth of field
• Moken youngsters can both accommodate and constrict their pupils to the utmost extent of human capacity, akin to adaptations observed in pinnipeds and cetaceans
• The absence of this capability in adult Moken suggests that with maturation, our lenses become less pliable and forfeit their ability to accommodate

Speculations on Moken Youngsters’ Underwater Sight ⁽⁵⁾

• One conjecture is that the capacity to see distinctly underwater may have evolved as an inherited characteristic
• An alternative possibility is that the capacity to accommodate underwater is an incidental result of the diving response, as the identical parasympathetic nerves regulate both this reflex and pupil constriction
• Other human populations with lifestyles akin to the Moken, such as the Bajau in the Philippines ⁽⁶⁾ and the Ama in Korea and Japan ⁽⁷⁾, have been noted to possess an exceptionally potent diving reflex
• Irrespective of its genesis, this behaviour in Moken youngsters is manifestly a beneficial adaptation
• It is postulated that with practice, this capacity may be achievable by other young individuals

 

References

1. Luria, S. M.; Kinney, J. A. (December 1974). “Linear polarizing filters and underwater vision”. Undersea Biomedical Research. 1 (4): 371–8. PMID4469103. Archived from the original on September 14, 2009. Retrieved 2008-07-06.
2. Adolfson, J.; Berghage, T. (1974). Perception and Performance Under Water. John Wiley & Sons. ISBN0-471-00900-8.
3. Wood, R. W. (1906-08-01). “XXIII. Fish-eye views, and vision under water”. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. 12 (68): 159–162. doi:1080/14786440609463529. ISSN1941-5982
4. “How Mokens see clearly underwater, Building Your Brain, Inside the Human Body – BBC One”. BBC. Retrieved 4 May 2018
5. Gislén, A., Dacke, M., Kröger, R. H. ., Abrahamsson, M., Nilsson, D.-E., & Warrant, E. J. (2003). Superior Underwater Vision in a Human Population of Sea Gypsies. Current Biology, 13(10), 833–836. doi:10.1016/s0960-9822(03)00290-2
6. Scholander, P.F., LeMessurier, H., Hemmingsen, E., Garey, W., and Hammel, H.T. (1962). Circulatory adjustment in pearl divers. J. Appl. Physiol. 17, 184–190
7. Hong, S.K., and Rahn, H. (1967). Diving women of Korea and Japan. Sci. Am. 216, 34–43. 

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