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.^[1][2]

Fish[link]

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[link]

By wearing a flat diving mask, humans can see clearly under water.^[1][2][3] 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 (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.^[4]

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.^[1] Also pincushion distortion and lateral chromatic aberration are noticeable. Double-dome masks restore natural sized underwater vision and field of view, with certain limitations.^[1]

Color vision[link]

Water is responsible for the attenuation of light due to absorption.^[1] 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.

The best colors to use for visibility in water was shown by Luria et al. and quoted from Adolfson and Berghage below:^[1][2]

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[link]

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]. 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.^[5] This ability appears to be due to training rather than evolution or genetics.^[6] 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. ^[7]

See also[link]

Underwater diving portal

• Snell's law

References[link]

Further reading[link]

• 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). http://www.clspectrum.com/article.aspx?article=100720. Retrieved 2009-06-27. 

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For the researcher at Woods Hole Oceanographic Institution, see Hanumant Singh (scientist).

Hanumant Singh  pronunciation (help·info) (29 March 1939 – 29 November 2006) was an Indian cricketer. He played in 14 Tests for the Indian cricket team from 1964 to 1969. He was later an International Cricket Council match referee in 9 Tests and 54 One Day Internationals from 1995 and 2002.

Singh was born in Banswara, Rajasthan. He was the second son of Chandraveer Singh, Maharawal of Banswara from 1944 to 1985, making him Maharajkumar of Banswara. His mother was the sister of Kumar Shri Duleepsinhji, making him the grandnephew of Kumar Shri Ranjitsinhji. His older brother, Suryaveer Singh, also played first-class cricket, as did his son, Sangram Singh. A cousin, KS Indrajitsinhji, also played in 4 Tests for India.

He played domestic first-class cricket for Madhya Bharat and then Rajasthan and Central Zone, and was known as "Chhotu" as a result of his short stature. He batted well from the back foot, particularly working the ball on the leg side. He made his Test debut in the 4th Test against England at Delhi in February 1964, scoring 105 and so becoming the fifth Indian to make a Test century on debut, emulating Lala Amarnath, Deepak Shodhan, AG Kripal Singh and Abbas Ali Baig. Later that year, he reached 94 in his first Test against Australia, out of a total of 193. He also played at home against New Zealand in 1964-5 and against West Indies in 1966-7, and toured England in 1967. However, like many other prominent Indian players, he was surprisingly excluded from the 1967-8 tour to Australia. Recalled to play against New Zealand at Bombay in September 1969, he scored 1 and 13, caught behind both times off the fast bowling of Dayle Hadlee, and did not play Test cricket again. He never scored a second Test century.

He continued to perform well in Indian domestic cricket. He was captain of Rajasthan in three Ranji Trophy finals, but lost each time. He also captained Central Zone to its first victory in the Duleep Trophy in 1971-72. In the Ranji Trophy final in 1966-67, he scored 109 and 213* against Bombay. His older brother, Suryaveer Singh, made 79 and 132 in the same match, and they shared partnerships of 176 and 213. Hanumant Singh retired from first-class cricket in 1979.

He was manager of the Indian team that toured the West Indies in 1983, and he served as an International Cricket Council match referee in 9 Tests and 54 ODIs from March 1995 to February 2002. He was also chairman of the National Cricket Academy, based in Bangalore, and a coach for Rajasthan. Outside cricket, he was an executive for State Bank of India.

Hanumant Singh died in Mumbai of lung and kidney failure, after contracting from dengue fever and hepatitis B.

References[link]

• Player profile: Hanumant Singh from ESPNcricinfo
• "Former India batsman Hanumant Singh dies aged 67", Reuters, 29 November 2006
• Hanumant Singh - the tragic Prince of Indian cricket, CricketArchive, 29 November 2006
• Obituary, The Daily Telegraph, 29 November 2006
• Obituary, The Times, 18 December 2006