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 (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.cite book |author=Adolfson J and Berhage, T |title=Perception and Performance Under Water |publisher=John Wiley & Sons |year=1974 |isbn=0471009008 ] cite journal |author=Luria SM, Kinney JA |title=Underwater vision |journal=Science (journal) |volume=167 |issue=924 |pages=1454–61 |year=1970 |month=March |pmid=5415277 |doi= |url=http://www.sciencemag.org/cgi/pmidlookup?view=long&pmid=5415277 |accessdate=2008-07-06]

Fish

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.

Masks and Goggles

decrease visibility underwater by limiting ambient light and dimming artificial light sources. [cite journal |author=Luria SM, Kinney JA |title=Linear polarizing filters and underwater vision |journal=Undersea Biomed Res |volume=1 |issue=4 |pages=371–8 |year=1974 |month=December |pmid=4469103 |doi= |url=http://archive.rubicon-foundation.org/2668 |accessdate=2008-07-06]

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.

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

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 items [http://www.cdnn.info/news/article/a040514.html] . Gislén et al. have compared Moken and European children and found that the visual acuity of the Moken was twice that of their European counterparts. [cite journal |author=Gislén A, Dacke M, Kröger RH, Abrahamsson M, Nilsson DE, Warrant EJ |title=Superior underwater vision in a human population of sea gypsies |journal=Curr. Biol. |volume=13 |issue=10 |pages=833–6 |year=2003 |month=May |pmid=12747831 |doi= |url=http://linkinghub.elsevier.com/retrieve/pii/S0960982203002902 |accessdate=2008-07-06] The good news for non-Moken people is that it seems that children can train to achieve better vision underwater without using goggles. [cite journal |author=Gislén A, Warrant EJ, Dacke M, Kröger RH |title=Visual training improves underwater vision in children |journal=Vision Res. |volume=46 |issue=20 |pages=3443–50 |year=2006 |month=October |pmid=16806388 |doi=10.1016/j.visres.2006.05.004 |url=http://linkinghub.elsevier.com/retrieve/pii/S0042-6989(06)00236-7 |accessdate=2008-07-06]

See also

Snell's law

References