Underwater vision
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Light rays bend when they enter from one medium to another of different densities making it possible to form images on the retina of our eyes by focussing the rays on the retina. If both the media have almost the same refractive indices (which determines the extent of bending), light rays would not 'bend properly' to be focussed on the retina of our eyes. The refractive index of water (1.34) and the refractive index of the transparent media of the human eye are almost the same, i.e. 1.34 for the vitreous humour and cornea; 1.43 for the crystalline lens, and 1.34 for the aqueous humour. As the refractive indices of the different elements of the eyes are identical with water, and that of the lens is only minimally greater than water, the rays focus far behind the retina resulting in an extremely blurred image.
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[edit] Fish
The crystalline lenses of the fishes' eyes are extremely convex, almost spherical, and its refractive index is highest of all the animals which enables 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 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
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 items1. Biologist Anna Gislén has compared Moken and European children2 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 you can train to achieve better vision underwater without using goggles3.
[edit] References
1. [1] (general article) 2. 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 3. [2]
