On a hot summer day there is nothing more refreshing than diving into the ocean, lake, river or swimming pool (remembering to check that the water is deep enough first). Once we are underwater our vision becomes blurry because our eyes are adapted to seeing light in air not in water.

Light travels through transparent mediums at different speeds.  The speed is measured by an index of refraction, which compares the speed of light through a certain medium compared to the speed of light through a vacuum. Light is hardly slowed when it travels through air consequently the index of refraction in air is 1.000277, but light is slowed down far more when it travels through the denser medium of water so the index for water is 1.33.

In air, our cornea refracts light, and the lens of our eyes changes shape, curves and uses refraction to bend and concentrate light onto a single focal point on the retina in the back of our eyes. This allows us to focus on an object.

Underwater, the index of the cornea and the index of water are almost equal, so the cornea and lens lose their refractive power and are virtually ineffective. Instead of light being concentrated in one spot on the retina it is projected into a large area on the retina. Vision is blurred because images are unfocused and overlap.

Goggles and masks provide clearer vision underwater because there is a layer of air between eyes and water, however there is still a certain amount of distortion which make objects appear closer and larger.

Fish eyes are adapted to seeing light in water. Although their eyes have the same basic structure as terrestrial animals, their eyes are perfectly spherical, crystalline, denser and have a higher refractive index than water to enable focusing to occur. Instead of focusing by stretching the eye lens as we do, fish focus by moving the lens towards or away from the retina like a camera.

Some species have amphibious vision which function in air and water. The main problem with amphibious vision is the loss of refractive power between air and water. There are a number of adaptations to overcome this problem. Seals have very large round eyes with pupils that can dilate widely or constrict into vertical slits; penguins have flattened corneas; dolphins and whales have two specialised areas in their retinas; some diving birds have powerful eye muscles which when underwater, can push the soft lens through the pupil into a bulge; other species of birds have transparent eyelids which covers the eye underwater like goggles.

Perhaps the most impressive example of adaptation for amphibious vision is a genus of mud dwelling fish called Anableps which has two sets of pupils in each eye, the lower for water and the upper for air allowing it to see simultaneously above and below the water surface.