Light rays bend when they travel from one medium to another; 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.
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. This convex lens gives the name to the fisheye lens in photography.
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. Linear polarizing filters decrease visibility underwater by limiting ambient light and dimming artificial light sources.
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.
Color vision 
Water is responsible for the attenuation of light due to absorption. In other words, as we go deeper on a dive, more color is absorbed by the water. Color vision is also affected by turbidity of the water as well as particulate matter.
A. For murky, turbid water of low visibility (rivers, harbors, etc.) 1. With natural illumination: a. Fluorescent yellow, orange, and red. b. Regular yellow, orange, and white. 2. With incandescent illumination: a. Fluorescent and regular yellow, orange, red and white. 3. With a mercury light source: a. Fluorescent yellow-green and yellow-orange. b. Regular yellow and white. B. For moderately turbid water (sounds, bays, coastal water). 1. With natural illumination or incandescent light source: a. Any fluorescent in the yellows, oranges, and reds. b. Regular yellow, orange, and white. 2. With a mercury light source: a. Fluorescent yellow-green and yellow-orange. b. Regular yellow and white. C. For clear water (southern water, deep water off shore, etc.). 1. With any type of illumination fluorescent paints are superior. a. With long viewing distances, fluorescent green and yellow-green. b. With short viewing distances, fluorescent orange is excellent. 2. With natural illumination: a. Fluorescent paints. b. Regular yellow, orange, and white. 3. With incandescent light source: a. Fluorescent paints. b. Regular yellow, orange, and white. 4. With a mercury light source: a. Fluorescent paints. b. Regular yellow, white. The most difficult colors at the limits of visibility with a water background are dark colors such as gray or black.
Biological variations 
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. Scuba divers with interest in underwater photography may notice presbyopic changes while diving before they recognize the symptoms in their normal routines due to the near focus in low light conditions.
The Moken people of South-East Asia are able to focus underwater to pick up tiny shellfish and other food items. Gislén et al. have compared Moken and European children and found that the underwater visual acuity of the Moken was twice that of their European counterparts. This ability appears to be due to training rather than evolution or genetics. This is due to the contraction of the pupil, instead of the usual dilation (mydriasis) that is undergone when a normal, untrained eye, accustomed to viewing in air, is submerged. 
See also 
- Adolfson J and Berghage, T (1974). Perception and Performance Under Water. John Wiley & Sons. ISBN 0-471-00900-8.
- Luria SM, Kinney JA (March 1970). "Underwater vision". Science 167 (3924): 1454–61. doi:10.1126/science.167.3924.1454. PMID 5415277. Retrieved 2008-07-06.
- Weltman G, Christianson RA, Egstrom GH (October 1965). "Visual fields of the scuba diver". Hum Factors 7 (5): 423–30. PMID 5882204.
- Luria SM, Kinney JA (December 1974). "Linear polarizing filters and underwater vision". Undersea Biomed Res 1 (4): 371–8. PMID 4469103. Retrieved 2008-07-06.
- Bennett QM (June 2008). "New thoughts on the correction of presbyopia for divers". Diving Hyperb Med 38 (2): 163–4. PMID 22692711. Retrieved 2013-04-19.
- Moken Sea Gypsies: Seeing Underwater at the Wayback Machine (archived August 29, 2008)
- Gislén A, Dacke M, Kröger RH, Abrahamsson M, Nilsson DE, Warrant EJ (May 2003). "Superior underwater vision in a human population of sea gypsies". Curr. Biol. 13 (10): 833–6. doi:10.1016/S0960-9822(03)00290-2. PMID 12747831. Retrieved 2008-07-06.
- Gislén A, Warrant EJ, Dacke M, Kröger RH (October 2006). "Visual training improves underwater vision in children". Vision Res. 46 (20): 3443–50. doi:10.1016/j.visres.2006.05.004. PMID 16806388. Retrieved 2008-07-06.
Further reading 
- Chou, B; Legerton, JA; Schwiegerling, J. "Improving Underwater Vision: Contact lenses and other options can help patients safely maximize their vision underwater". Contact Lens Spectrum (June 2007). Retrieved 2009-06-27.