Tuesday, December 16, 2008

Underwater vision

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, 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.

scuba mask
Photo: scuba.about.com

Masks and Goggles Views through a flat mask, above and below water. 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.[1] 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 in a manner similar to viewing fish kept in an aquarium. Linear polarizing filters decrease visibility underwater by limiting ambient light and dimming artificial light sources.

fish's eye
Photo: kac-divers.com

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.

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