Diving Deeper: Why Our Eyes Are Not Meant for The Deep Blue?

Mr Vivek Suganthan R, M. Optom

Assistant Professor

 

Dr. Aiswaryah Radhakrishnan, PhD

Associate Professor, Department of Optometry, SRM Medical College Hospital and Research Centre, Chennai, India

 

Underwater environment:

Our fascination with the underwater world is undeniable. But for humans, venturing beneath the waves comes with a significant visual hurdle. Our eyes, perfectly tuned for the terrestrial environment, struggle to function underwater due to fundamental differences in how light travels. ⁽¹⁾

The crux of the issue lies in refraction. The cornea, responsible for two-thirds of our eye’s focusing power on the air, loses its dominance underwater. ⁽²⁾ This happens because the refractive index of water is like that of the cornea, negating its focusing ability and resulting in a blurry image. A study by Cramer et al., ⁽³⁾ suggests that uncorrected myopes have an advantage underwater due to their corneas already compensating for some refractive power when compared to emmetropes.

For swimmers, divers, and anyone working underwater, this blur translates to hampered productivity, navigation, and safety concerns. Reduced illumination is another challenge. ⁽⁴⁾ As we descend, light scatters, making everything dimmer. ^[4] Water’s wavelength absorption further complicates matters, filtering out colours and limiting the visible spectrum. ⁽⁵⁾

Vision in water:

Research by Atchison et al., ⁽⁶⁾ quantified this visual decline. They measured visual acuity, and contrast sensitivity by simulating a water environment with goggles filled with saline solution. All these functions suffered significantly underwater, with a mean loss of over 1 log MAR in acuity and a 2-log unit drop in contrast sensitivity compared to air.

However, a glimmer of hope emerges from the Moken people, sea nomads who spend a large part of their lives underwater. Studies by Gislen et al., ⁽²⁾ revealed that Moken children possess superior underwater vision (twice as good as European children) due to a larger pupil constriction. This adaptation allows them to gather more light in the dim underwater environment.

Effect of training:

Interestingly, another study by the same investigators ⁽⁷⁾ showed that after a month of underwater training, European children achieved similar visual acuity and pupil constriction, as that of Moken children suggesting that our eyes have a degree of adaptability. Beyond acuity and contrast, Radhakrishnan et al., ⁽⁸⁾ have found that underwater blur also affects vernier acuity and fixational eye movements.

Current solution:

Thankfully, technology has come to the rescue. Flat diving masks create an air pocket between the eye and the water, allowing light rays to travel through a familiar medium(air) and refocus on the retina. ^(5,9,10) This mechanism counteracts the refractive index mismatch between the cornea and water, which typically worsens underwater vision. However, these masks come with their slight disadvantage. Objects appear closer and larger underwater (around 25% and 33% respectively), and there can be some distortion around the edges of the field of view. ⁽⁵⁾

Double-dome masks attempt to rectify these issues by offering a more natural underwater view. ^(9,11) For divers needing vision correction, specially designed masks with corrective lenses can be used. These lenses are flat on one side to fit within the mask and provide consistent correction above and below the surface.

Conclusion:

While our eyes may not be naturally suited for the underwater world, a combination of biological adaptation, training, and technology allows us to explore the depths with greater clarity. So, the next time you dive in, remember the fascinating science behind the scenes that lets you see the wonders hidden beneath the waves.

Declaration: The blog is written solely for education purposes, and it does not have any financial support or conflict of interest

 

References:

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2. Gislen A, Dacke M, Kroger RHH, Abrahamsson M, Nilsson Dan-Eric, Warrent EJ. Superior underwater vision in a human population of sea gypsies. Current Biology 2003;13:833– 6.
3. Cramer JL. Comparative analysis of unaided emmetropic and unaided noncorrected myopic underwater visual acuity. Research Quarterly of the American Alliance for Health, Physical Education and Recreation 1975;46(1):100–9.
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11. Sawatzky D. Corrective Dive Masks. DIVER Editorial 2015.