question: Why does everything look blurry when we look underwater even though our eyes are normally wet anyway?
comment: Because the lens in the eye is designed to focus through lens = > water = > air layers. Without the air layer, the lens is incorrect because it depends on the curved water = > air boundary that not present when you're in water.
passages: 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. The human eye is 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 immersed in water, instead of focusing images on the retina, they are focused behind the retina, resulting in an extremely blurred image from hypermetropia.
   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), so immersion effectively eliminates the cornea's focusing properties. When our eyes are in water, instead of focusing images on the retina, they now focus them far behind the retina, resulting in an extremely blurred image from hypermetropia.
   Section::::Focus.
  
  Water has a significantly different refractive index to air, and this affects the focusing of the eye. Most animals' eyes are adapted to either underwater or air vision, and do not focus properly when in the other environment.
  
  Section::::Focus.: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. This convex lens gives the name to the fisheye lens in photography.
  
  Section::::Focus.:Humans.
   Water in the eye can alter the optical properties of the eye and blur vision. It can also wash away the tear fluid—along with it the protective lipid layer—and can alter corneal physiology, due to osmotic differences between tear fluid and freshwater. Osmotic effects are made apparent when swimming in freshwater pools, because the osmotic gradient draws water from the pool into the corneal tissue (the pool water is hypotonic), causing edema, and subsequently leaving the swimmer with "cloudy" or "misty" vision for a short period thereafter. The edema can be reversed by irrigating the eye with hypertonic saline which osmotically draws the excess water out of the eye.
   A very near-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 underwater. 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.
   Where R is the recharge rate in meters, p is precipitation (m), and ET is evapotranspiration (m) of water. With higher amounts of recharge, the hydraulic head is increased and a thick freshwater lens is maintained through the dry season. Lower rates of precipitation or higher rates of interception and evapotranspiration will decrease the hydraulic head resulting in a thin lens.
  
  Section::::Models of freshwater lenses.
  
  Section::::Models of freshwater lenses.:Algebraic model.
   As opposite to air, water attenuates light exponentially. This results in hazy images with very low contrast. The main reasons for light attenuation are light absorption (where energy is removed from the light) and light scattering, by which the direction of light is changed. Light scattering can further be divided into forward scattering, which results in an increased blurriness and backward scattering that limits the contrast and is responsible for the characteristic veil of underwater images. Both scattering and attenuation are heavily influenced by the amount of organic matter dissolved in the water.
   Section::::Equipment.:Underwater vision.
  
  Water has a higher refractive index than air – similar to that of the cornea of the eye. Light entering the cornea from water is hardly refracted at all, leaving only the eye's crystalline lens to focus light. This leads to very severe hypermetropia. People with severe myopia, therefore, can see better underwater without a mask than normal-sighted people. 
   The depth that the water appears to be when viewed from above is known as the "apparent depth". This is an important consideration for spearfishing from the surface because it will make the target fish appear to be in a different place, and the fisher must aim lower to catch the fish. Conversely, an object above the water has a higher "apparent height" when viewed from below the water. The opposite correction must be made by an archer fish.
   BULLET::::- Snell's law - the index of refraction of water is similar to that of the cornea of the eye—30% greater than air. This is the reason a diver cannot see clearly underwater without a diving mask with an internal airspace.
  
  Section::::Physical characteristics of water most relevant to divers.
  
  The physical effects of water or the underwater environment are:
  
  BULLET::::- Pressure - the overall pressure on a diver is the sum of the local atmospheric pressure and hydrostatic pressure.
   Section::::Color vision.
  
  Water attenuates light due to absorption which varies as a function of frequency. In other words, as light passes through a greater distance of water color is selectively absorbed by the water. Color absorption is also affected by turbidity of the water and dissolved material.
   The refractive index of human lens varies from approximately 1.406 in the central layers down to 1.386 in less dense layers of the lens. This index gradient enhances the optical power of the lens.
  
  Aquatic animals must rely entirely on their lens for both focusing and to provide almost the entire refractive power of the eye as the water-cornea interface does not have a large enough difference in indices of refraction to provide significant refractive power. As such, lenses in aquatic eyes tend to be much rounder and harder.
  
  Section::::Function.:Crystallins and transparency.
   Over/under photos necessitate the lens or port to be partly below and partly above the surface. When bringing the outer optical surface out of the water, droplets can be left on the surface which can distort the image. This can be avoided to some extent by wiping off the droplets with a chamois leather cloth above the water and lowering the camera to working position. Keeping the port fully wet is an alternative option, which requires the shot to be taken before the water on the top part of the lens surface separates into droplets. Which approach works better will depend on the surface tension of water on the lens surface.
   With macro lenses, the distortion caused by refraction is not a problem, so normally a simple flat glass port is used. Refraction increases the magnification of a macro lens; this is considered a benefit to photographers who are trying to capture very small subjects.
  
  Section::::Underwater flash.
   As mentioned above, a refractive cornea is only useful out of water. In water, there is little difference in refractive index between the vitreous fluid and the surrounding water. Hence creatures that have returned to the water—penguins and seals, for example—lose their highly curved cornea and return to lens-based vision. An alternative solution, borne by some divers, is to have a very strongly focusing cornea.
  
  Section::::Types.:Non-compound eyes.:Reflector eyes.
   A diving mask (also half mask, dive mask or scuba mask) is an item of diving equipment that allows underwater divers, including scuba divers, free-divers, and snorkelers, to see clearly underwater. Surface supplied divers usually use a full face mask or diving helmet, but in some systems the half mask may be used. When the human eye is in direct contact with water as opposed to air, its normal environment, light entering the eye is refracted by a different angle and the eye is unable to focus the light on the retina. By providing an air space in front of the eyes, the eye is able to focus nearly normally. The shape of the air space in the mask slightly affects the ability to focus. Corrective lenses can be fitted to the inside surface of the viewport or contact lenses may be worn inside the mask to allow normal vision for people with focusing defects.
   Lenses are normally spherical but can be slightly elliptical in some species. Compared to terrestrial vertebrates, fish lenses are generally more dense and spherical. In the aquatic environment there is not a major difference in the refractive index of the cornea and the surrounding water (compared to air on land) so the lens has to do the majority of the refraction. Due to "a refractive index gradient within the lens — exactly as one would expect from optical theory", the spherical lenses of fish are able to form sharp images free from spherical aberration.
   An algebraic model for estimating the thickness of a freshwater lens was developed using groundwater simulations by Bailey et al. 2008. This equation relates lens thickness to geologic and climatic factors such as island geometry, geologic composition, and recharge rate, among others. The equation is summarized below:
  
  formula_1
   Many freshwater aquifers on atolls and small rounded islands take on the form of a Badon Ghyben-Herzberg Lens. This relationship is described in the equation below:
  
  formula_11
  
  Where H = the depth of the lens below sea level, formula_12 = the density of the freshwater aquifer, formula_13= density of saltwater, and formula_14 = thickness of lens above sea level.
  
  Section::::Effects of drought.
   Underwater, things are less visible because of lower levels of natural illumination caused by rapid attenuation of light with distance passed through the water. They are also blurred by scattering of light between the object and the viewer, also resulting in lower contrast. These effects vary with wavelength of the light, and color and turbidity of the water. The vertebrate eye is usually either optimised for underwater vision or air vision, as is the case in the human eye. The visual acuity of the air-optimised eye is severely adversely affected by the difference in refractive index between air and water when immersed in direct contact. Provision of an airspace between the cornea and the water can compensate, but has the side effect of scale and distance distortion. The diver learns to compensate for these distortions. Artificial illumination is effective to improve illumination at short range.
   Underwater vision is affected by the clarity and the refractive index of the medium. Visibility underwater is reduced because light passing through water attenuates rapidly with distance, leading to lower levels of natural illumination. Underwater objects are also blurred by scattering of light between the object and the viewer, resulting in lower contrast. These effects vary with the wavelength of the light, and the colour and turbidity of the water. The human eye is optimised for air vision, and when it is immersed in direct contact with water, visual acuity is adversely affected by the difference in refractive index between water and air. Provision of an airspace between the cornea and the water can compensate, but causes scale and distance distortion. Artificial illumination can improve visibility at short range. Stereoscopic acuity, the ability to judge relative distances of different objects, is considerably reduced underwater, and this is affected by the field of vision. A narrow field of vision caused by a small viewport in a helmet results in greatly reduced stereoacuity, and an apparent movement of a stationary object when the head is moved. These effects lead to poorer hand-eye coordination.
   Diving masks and helmets solve this problem by providing an air space in front of the diver's eyes. The refraction error created by the water is mostly corrected as the light travels from water to air through a flat lens, except that objects appear approximately 34% bigger and 25% closer in water than they actually are. The faceplate of the mask is supported by a frame and skirt, which are opaque or translucent, therefore total field-of-view is significantly reduced and eye–hand coordination must be adjusted.
   In order to be able to use a camera in the water, a watertight housing is required. However, refraction will happen at the water-glass and glass-air interface due to differences in density of the materials. This has the effect of introducing a non-linear image deformation.
  
  The motion of the vehicle presents another special challenge. Underwater vehicles are constantly moving due to currents and other phenomena. This introduces another uncertainty to algorithms, where small motions may appear in all directions. This can be specially important for video tracking. In order to reduce this problem image stabilization algorithms may be applied.
   Section::::Frequent Methods.
  
  Section::::Frequent Methods.:Image restoration.
  
  Image restoration aims to model the degradation process and then invert it, obtaining the new image after solving. It is generally a complex approach that requires plenty of parameters that vary a lot between different water conditions.
  
  Section::::Frequent Methods.:Image enhancement.
  
  Image enhancement on the opposite only tries to provide a visually more appealing image without taking the physical image formation process into account. These methods are usually simpler and less computational intensive.
  
  Section::::Frequent Methods.:Color correction.
   Underwater, things are less visible because of lower levels of natural illumination caused by rapid attenuation of light with distance passed through the water. They are also blurred by scattering of light between the object and the viewer, also resulting in lower contrast. These effects vary with wavelength of the light, and color and turbidity of the water. The vertebrate eye is usually either optimised for underwater vision or air vision, as is the case in the human eye. The visual acuity of the air-optimised eye is severely adversely affected by the difference in refractive index between air and water when immersed in direct contact. provision of an airspace between the cornea and the water can compensate, but has the side effect of scale and distance distortion. Artificial illumination is effective to improve illumination at short range.
  
presuppositions: Things should not look blurry when looking underwater because our eyes are normally wet. Things should not look blurry when looking underwater because our eyes are normally wet. Water causes blurry vision. Since eyes are normally wet, vision should not be blurry underwater.
corrections: The lens of the eye is incorrect when underwater and things look blurry because there is no air layer. The lens of the eye is incorrect when underwater and things look blurry because there is no air layer. Blurry vision is caused by the air boundary not being present underwater. The lens in the eye is designed to focus through both water and air, so without air the lens does not focus correctly.
raw_presuppositions: Things should not look blurry when looking underwater because our eyes are normally wet. Things should not look blurry when looking underwater because our eyes are normally wet. Water causes blurry vision. Since eyes are normally wet, vision should not be blurry underwater. Things should not look blurry when looking underwater because our eyes are normally wet. Things should look clear, not blurry, when under water because our eyes are normally wet.
raw_corrections: The lens of the eye is incorrect when underwater and things look blurry because there is no air layer. The lens of the eye is incorrect when underwater and things look blurry because there is no air layer. Blurry vision is caused by the air boundary not being present underwater. The lens in the eye is designed to focus through both water and air, so without air the lens does not focus correctly. The lens of the eye is incorrect when underwater and things look blurry because there is no air layer. The lens focus is not the same under water without air layer.