Underwater vision
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Light rays bend when they enter from one medium to another of different density. The amount of bending is determined by the refractive indices of the two media. If one medium has a particular curved shape, it functions as a lens. The cornea, humours, and crystalline lens of the eye together form a lens that focuses images on the retina. Our eyes are adapted for viewing in air. Water, however, has approximately the same refractive index as the cornea (both about 1.33), effectively eliminating the cornea's focusing properties. When our eyes are in water, instead of their focusing images on the retina, they now focus them far behind the retina, resulting in an extremely blurred image from hypermetropia.
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[edit] Fish
The crystalline lenses of fishes' eyes are extremely convex, almost spherical, and their refractive indices are the highest of all the animals. These properties enable proper focusing of the light rays and in turn proper image formation on the retina.
[edit] Masks and Goggles
By wearing a flat diving mask, humans can see clearly under water. The scuba mask's flat window separates one's eyes from the surrounding water by a layer of air. Light rays entering from water into the flat parallel window change their direction minimally within the window material itself. But when these rays exit the window into the air space between the flat window and the eye, the refraction is quite noticeable. The view paths refract (bend) in a manner similar to viewing fish kept in an aquarium.
While wearing a flat scuba mask or goggles, objects underwater will appear 33% bigger (34% bigger in salt water) and 25% closer than they actually are. Also pincushion distortion and lateral chromatic aberration are noticeable. Double-dome masks restore natural sized underwater vision and field of view, with certain limitations.
[edit] Trivia
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A very short-sighted person (eyesight abnormality resulting from the eye's faulty refractive index due to which the distant objects appear blurred) can see more or less normally under water.
The Moken people of South-East Asia are able to focus underwater to pick up tiny shellfish and other food items [1]. Biologist Anna Gislén has compared Moken and European children [1] and found that the visual acuity of the Moken was twice that of their European counterparts. The good news for non-Moken people is that it seems that some people can train to achieve better vision underwater without using goggles [2].
[edit] References
- ^ Anna Gislén, Marie Dacke, Ronald H.H. Kröger, Maths Abrahamsson, Dan-Eric Nilsson, and Eric J. Warrant, Superior Underwater Vision in a Human Population of Sea Gypsies, Current Biology 2003 13: 833-836
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