If one can see more clearly underwater without goggles, are they near-sighted, far-sighted, or neither. Why?

Question

thanks =D

Answer 1

Nearsighted:
Consider the following taken from wikipedia (pay attention to trivia at the end). It all has to do with light bending as it enters and exits different media ie water to air and air to water. The eye itself is close enough to water as to consider it the same as water.

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.


[edit] Fish
The crystalline lenses of the fishes' eyes are extremely concave and its refractive index is highest of all the animals which enables proper focussing of the light rays and in turn proper image formation on the retina.


[edit] Goggles
By wearing a flat waterproof glass mask, humans can see under water. The flat waterproof glassmask separates the human eyes from the surrounding water by a layer of air. Light rays entering from water to the flat parallel glass don't change their direction under the water. But when they enter from air (the waterproof layer of air between the flat piece of glass and the eye) into the eye (separated by the thin flat piece of glass), they just refract as naturally as they refract (bend) over the water. Viewing of fishes kept in the aquarium can be explained similarly.

A Glass Mask (for example under water goggles) however cannot eliminate all the distortion under water; even while wearing a glass mask, all objects under water will appear 25% bigger and 25% closer than they actually are.


[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.

Retrieved from "http://en.wikipedia.org/wiki/Underwater_vision"

Answer 2

Mermaids probably don't exist. But there are children of certain tribes in Asia who are distinctly fishlike.
Called sea gypsies, these island-dwelling people are famous for their swimming and diving skills. A study now shows that sea-gypsy children spend so much time swimming that their eyes have adapted especially well to seeing clearly underwater.

Answer 3

I think anyone can learn to see clearly underwater-it doesn't have anything to do with near- or farsightedness.

Answer 4

Neither. It is not possible to see under water with your naked eye.